Contact
instrument.games is built and maintained by Josh Stovall. Email is the only channel — there is no support queue behind it, so a reply comes from the same person who wrote the thing you are writing about.
Worth writing about: a level that scores wrong, a MIDI keyboard or microphone the input picker will not see, a leaderboard entry that should not be there, or a drill you want that does not exist yet. Bug reports are more useful with the game, the level, and the browser you were in.
Requests to access or delete your account data go to the same address, as described in the privacy policy.