Support for "unkeyed" (but not unpitched) instruments, needed for timpani and historic brass
See https://musescore.org/en/node/154611#comment-629151. Some instruments are normally written without key signatures, even in a score among others that do use them.
I have caught myself forgetting to add the Open/Atonal key signature specifically to the timpani staff at every key change. This should be just a property of the instrument.