It would probably be an unfair labour practice for Twitter to do so, as it's pretty clear that twitter employees can use twitter at will and such a policy would be purely suppressing their organizing.

Of course twitter is a terrible medium for organizing anyway, so maybe Twitter would want them to use it! I hope if twitter employees do try to unionize they don't try and do it in tweets.

I guess I’m curious whether Twitter would be compelled to allow tweets that “violate” its tos.

Let’s say it’s a rally: “Let’s organize and prevent Twitter from requiring proof of immunization status.”

It’s about unionization and demanding that Twitter not demand proof of immunization (if they were to require it), but it could be any number of things that are contrary to their liking including unionization.