Chat verification should be possible without mobil number verification, if a streamer trust a user. Otherwise lot's of people who do not want to give out their mobile number, could not be an active member of the community anymore. And the streamer and twich as well are benefit from active user, which are chatting and make the platform more interactive.
I understand that new and unverified account should not be able chat directly everywhere, but for twitch users which a following a streamer a very long time, it should be possible to allow them to chat.
So maybe the streamer shoud be able to decide by the chat settings if unverified (without mobile number) users should be allowed to chat in their streams. And in combination with some other settings e.g. Unverified chatting only allowed when follow at least X month. So the streamer can prevent new, unverified accounts from chatting, but enable reliable users to be still a part from their community.