I saw some recently. Will run a check and report back.
My mistake, I confused maximum characters vs. maximum bytes (Thanks @SimonPoole for clarifying!)
I ran a check on a recent planet file, all key and value strings are indeed 255 characters or less.
Since a UTF-8 character can be 1 to 4 bytes, a legal OSM string can occupy a maximum of 1020 bytes (not including length counter or any terminating character).
Or, in the opposite direction… Do diacritical marks count towards the character limit? (“é” could be single character, or two code points (base letter and the combining acute accent)