I’m not sure if this a bug or by design, so I wasn’t sure where to categorize the topic. I ended up going with feature though since this isn’t exactly preventing me from using Discourse normally (normally I don’t send > 500 character long flags).
When you flag > send message, you end up with a 500 character limit:
But when you PM someone from their profile, there isn’t a character limit, so I’m not sure why there’s one on the flag PMing. Maybe because all flags with custom text (send message and something else) have a character limit and no exception was added to send message? Regardless, it seems a bit odd that PMing someone from their profile doesn’t have a character limit, but PMing someone through a flag does.
I’m also not sure why flagging a post for “something else” needs a character limit. Moderators aren’t going to remove someone from the forums if I flag a users’ most recent post bashing the staff / parent site and tell them “Trust me, this guy has been doing this forever. He should be removed.” – the staff can’t just take my word for it, but if I submit a long list of posts of him doing this, that’s much more believable. Unfortunately, I can’t submit a long list of evidence with the character limit. If the “something else” character limit exists for UI reasons (bad user experience to be typing out > 500 character PMs in that small flag text box), it’d be nice if we could pop it out into a full editor. If the limit exists because “flags shouldn’t normally exceed 500 characters or else there’s too much fluff” or something like that, it’d be nice if that could be taken a look at again.