#Twitter just now released a "Promotion of alternative social platforms policy" that includes #Mastodon.
"We will no longer allow free promotion of certain social media platforms on Twitter… we will remove accounts created solely for the purpose of promoting other social platforms and content that contains links or usernames for the following platforms: #Facebook, #Instagram, Mastodon, Truth Social, Tribel, Nostr and #Post."
https://help.twitter.com/en/rules-and-policies/social-platforms-policy