Not most of the time anyway, I’ve seen a captcha from time to time that was case sensitive, but they’re uncommon for sure
It used to be that they paired a known word with an unknown word, and if you got the known word right you would pass no matter what you wrote for the unknown one.
I remember reading that somewhere (probably 4chan) figured out a somewhat high accuracy way to tell which was the control and which was the variable, and started spam solving them by correctly doing the assumed control but putting the same thing (knowing 4chan, probably a slur) for the assumed variable until the system got enough confirmation to move the word to the control rotation and started accepting the word for that. Can’t remember where so it may be unconfirmed
