I discovered (when answering this question) that string comparison in
filter is case insensitive: the formulas
=filter(A:A, B:B = "Yes") and
=filter(A:A, B:B = "YES") have the same output.
Apparently, this is true generally for string comparison in Google Sheets:
="Y"="y" returns TRUE, and so does its equivalent
This is convenient sometimes, but what to do when I want to filter rows by case-sensitive string equality?