It may very well be that Twitter removed mention of the policy that you can't link to a rival social media service from Twitter, but as of a minute ago, they haven't stopped preventing you from publishing tweets with a Mastodon link in it.
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@sysop408 you can maybe still use the site https://spacekaren.sucks/
to get a link to paste your mastodon #instance into Twitter #twitter #twittermigration
@mandaroza that indeed did work when I tried it earlier today.
About 20 minutes ago I just decided to see if they removed the initial restriction along with removing mention of that they were doing this in the Twitter Help.