I looked into it and here are the stats:
Since May 28th you can only chat on Twitch, if you have verified your telephone number. Twitch ignores the chat verification settings.
I reported this to Twitch, but I only got an useless automtic answer in response. As always.
Tested with several unverified accounts.
Sorry for my very late reply. I didn't expect an answer at all. That's why I only check Mastodon once in a while. 🙁
Those are not newish accounts.
And that bug appeared in a Stream while I was chatting. From one second to another I couldn't post anymore. I just got the message, that I need to verify the account with a telephone number. (That is untrue. I just needed to verfiy my email to chat again.)
It's also possible to write with an unverified account, if bits are used.