Capitalizing the First Letter of Sentences in a Cell

The Tip's SentenceCaseSelection macro includes the following statement:
            sText = Application.Trim(c.Value)
Notice Application.Trim is the same as Worksheet.Trim unless there is an error; in that case, the former simply returns the error value but the latter interrupts processing and displays an error message (unless modified by an On Error... statement). On the other hand, the macro's objective does not require use of Trim; therefore, the previous statement can be replaced with this statement
            sText = c.Value
The macro also includes a multiline ElseIf...And...Then... statement that does not contribute to the objective; therefore, it can be deleted.
As discussed in my most recent comment below, the Tip's macro might have some difficulty with abbreviations because their terminating period (.) is treated as the end of a sentence. For example, the following text
    pick a color (e.g., red, blue, etc.) then click OK. what did you pick?
yields this result
    Pick a color (e.G., red, blue, etc.) Then click OK. What did you pick?
and this text
    notice e.g. and i.e. have no space after a period (.) but etc. does.
yields this result
    Notice e.G. And i.E. Have no space after a period (.) But etc. Does.
One of my earlier comments discussed AutoCorrect Exceptions, which are generally abbreviations. Here's an alternate version of the Tip's macro that ignores AutoCorrect Exceptions when evaluating sentence ending punctuation.

Sub SentenceCaseSelection2()
    Dim rCell As Range, sPrev As String, sText As String, sChar As String
    Dim n As Integer, bCapNext As Boolean
    'enable Tools > References > Microsoft Scripting Runtime
    Static Exceptions As Scripting.Dictionary, bExcept As Boolean
    On Error Resume Next
        If Exceptions Is Nothing Then 'do this once; result is Static
            Set Exceptions = New Scripting.Dictionary
            With CreateObject("Word.Application")
                If Err = 0 Then 'Microsoft Word is available
                    With .AutoCorrect.FirstLetterExceptions
                        bExcept = (Err = 0 And .Count > 0)
                        If bExcept Then 'AutoCorrect Exceptions
                            For n = 1 To .Count 'add lower-case as key
                                Exceptions.Add LCase(.item(n).Name), Null
                                Err.Clear 'in case key is duplicated
                            Next n
                        End If
                    End With
                    Err.Clear
                    .Quit 'Word.Application
                End If
            End With
        End If
    On Error GoTo 0
    For Each rCell In Selection
        If rCell.HasFormula = False And VarType(rCell.Value) = vbString Then
            bCapNext = True 'capitalize the cell's first word
            sPrev = vbNullString
            sText = rCell.Value
            For n = 1 To Len(sText)
                sChar = Mid(sText, n, 1)
                If bCapNext And sChar Like "[a-z]" Then 'capitalize this
                    Mid(sText, n, 1) = UCase(sChar)
                    bCapNext = False
                ElseIf sChar Like "[.?!]" Then 'end of sentence
                    bCapNext = True
                End If
                If bExcept Then 'consider Exceptions (lower-case)
                    If sChar Like "[a-z.]" Then
                        sPrev = sPrev & sChar 'possible exception/abbreviation
                    Else
                        sPrev = vbNullString 'reset
                    End If
                    If sChar = "." Then 'check for exception/abbreviation
                        bCapNext = (Not Exceptions.Exists(sPrev))
                        sPrev = vbNullString 'reset
                    End If
                End If
            Next n
            rCell.Value = sText 'update the cell's text
        End If
    Next rCell
End Sub

With this macro the following text
    pick a color (e.g., red, blue, etc.) then click OK. what did you pick?
yields this result
    Pick a color (e.g., red, blue, etc.) then click OK. What did you pick?
and this text
    notice e.g. and i.e. have no space after a period (.) but etc. does.
yields this result
    Notice e.g. and i.e. have no space after a period (.) But etc. does.
The last example is not perfect, but it is close.

The Tip's four formulas are based on Ginny wanting "...to make sure that ONLY the first letter of each sentence in a cell is capitalized." As discussed in my most recent comment below, I believe Ginny ONLY wants to make sure that the first letter of each sentence in a cell is capitalized. I don't think she wants to disturb the remainder of that sentence. This relates to Ginny noting "...in Word, she can easily make sure that sentences start with a capital letter."
The four formulas in the Tip capitalize the first letter of a sentence but reduce the remainder to lower case. This can easily be corrected by removing LOWER from each formula. (The remainder of this comment assumes LOWER has been removed.)
Notice the third formula =LET(... fails when presented with an abbreviation like e.g. or i.e. because it assumes sentence-ending punctuation is always followed by at least one space character. For example, this text
    abbreviations like e.g. and i.e. have no space after a period (.).
returns nonsense (which was not accepted when this comment was first submitted). However, that problem can be corrected by changing this part of the formula
    {".","!","?"}
and adding a space character after each sentence-ending punctuation like this
    {". ","! ","? "}
Then this text
    abbreviations like e.g. and i.e. have no space after a period (.).
returns this result
    Abbreviations like e.g. And i.e. Have no space after a period (.).
For this example, both the corrected third formula =LET(... and the fourth formula =REGEXREPLACE(... treat an abbreviation followed by at least one space as the end of a sentence; therefore, they capitalize the next word. On the other hand, an abbreviation followed by a character other than space is not sentence-ending, and this text
    pick a color (e.g., red, blue, etc.) then click OK. what did you pick?
returns this result
    Pick a color (e.g., red, blue, etc.) then click OK. What did you pick?
In conclusion, the corrected =LET(... formula and the =REGEXREPLACE(... formula will sometimes return a satisfactory result when the text includes an abbreviation, but not always.

The AutoCorrect option in the Tip's step 4 and Figure 1 is labelled "Capitalize first letter of sentences" but enabling it actually capitalizes the first letter AFTER typing a completed sentence. So if you manually type this
    pick a color (e.g., red, blue, etc.) then click OK. what did you pick?
you will see this
    pick a color (e.g., red, blue, etc.) then click OK. What did you pick?
Notice only the W in What was capitalized. Abbreviations were ignored; these are listed when you click the Exceptions button seen in Figure 1 of the Tip to open the AutoCorrect Exceptions dialog (see Figure 1 below)
The Exceptions and several of the AutoCorrect options are shared with Word; therefore, I believe this feature is referenced by Ginny when she "...notes that in Word, she can easily make sure that sentences start with a capital letter."
My Excel Toolbox includes the following dynamic array function to list the status of all AutoCorrect, AutoFormat As You Type, and Math AutoCorrect options indicated by tabs in the Tip's Figure 1:
    =ListAutoCorrectOptions([AddExceptions])
If optional AddExceptions is TRUE, the list will include all of the AutoCorrect Exceptions; default is FALSE for a result with 12 rows in 1 column. Exceptions add more than 280 rows in my test.
See https://sites.google.com/view/MyExcelToolbox/

On my version of Excel (365 Family, Win 11), the last, built in option, works well for multiple sentences in a cell, except the first sentence in that cell. Am I missing something?

On the other hand the exceptions settings prevent capitalization after common abbreviations like ft. in. Jan. Feb. etc.
I would remove Mr. Mrs. Dr. Prof. from the exceptions as they are usually followed by a capitalized name.

Ginny wants "...to make sure that ONLY the first letter of each sentence in a cell is capitalized." This might have some unintended consequences with respect to abbreviations and names.

Re. the Tip's first two formulas:
If cell A1 contains: Tell Mr. Allen Wyatt Hello.
Each formula returns: Tell mr. allen wyatt hello.

Re. the Tip's last two formulas, abbreviations like Mr., Dr., i.e., etc., are treated as the end of a sentence:
If cell A1 contains: Tell Mr. Allen Wyatt Hello.
Each formula returns: Tell mr. Allen wyatt hello.

But those last two formulas work well if there are no abbreviations or names:
If cell A1 contains: Join Me. Are You Ready? All For One. Let's Go!
Each formula returns: Join me. Are you ready? All for one. Let's go!