There is! It just doesn't look like a #. ♭ is the flat symbol. There's also ♮ for 'natural', which means "even though this note would have a sharp or a flat in the key we're in, play it normally instead".
Oh! Yeah, most of the time I see a lowercase b used for it (e.g. C7 = C E G Bb).
@Felthry It's been a hash for me for as long as I can remember, certainly longer than I've been online.
We parse it as just the literal word 'number' most of the time if that counts. We have adapted to correcting it to 'hashtag' if we don't see a digit afterward, for better or for worse.
@Felthry
Number sign and pound sign both come to mind to me before "hashtag" does. Even just "hash" crops up before "hashtag". ♯, meanwhile, is "sharp".