Just verified with a Twitter test account:

Twitter is now blocking all links to well-known Mastodon instances (including mastodon.social and hachyderm.io) from being tweeted or added to profiles.

Best-case, a heuristic is broken, and it probably has no staff to run it anymore. That's my most optimistic theory, and even I'm not naive enough to believe it at this point.

“Free speech", my ass.

Freedom doesn't hide alternatives from you or try to prevent you from leaving.