But how would this look or behave in the context of a forum?
If you have direct messages between members, I could understand that you might want to prevent to write to a frozen user.
But I think every native Flarum feature interacts with users passively so there would be no difference in behavior between a user who stopped posting and a frozen user (?)
If users can unfreeze themselves, then I don't think this will offer any benefit for moderation (you can't assume a frozen user will no longer be causing trouble), and if they can't unfreeze themselves, it's not very different from the suspend feature in behavior?
In relation to notifications there was this earlier discussion https://discuss.flarum.org/d/25858-what-should-happen-if-mention-user-in-restrict-tag/7 where I mentioned the idea of showing to the user whether a given mention will be notified to the user. If freezing an account disables notifications, then this could be linked to that feature.