I'm not a Mastodon/etc. users but I can sympathize with some other sites. Even here has its own Ongoing September from redditors.
I would recommend reaching out to moderation teams and raising awareness, because they probably have far more ability to put global notifications or sign-up messages, and to give warnings to uncomfortable behaviour.
Make sure to call out twitter carryover, in a constructive way, so that people are aware that Mastodon isn't 'twitter but here'.