In my opinion, the solution needs to be a guarantee of transfering your account to another provider. We can do it with phone numbers, we can do it with banks (at least in Europe). I don't see a reason why we couldn't apply the same on social media and auth providers.

Banned from "login with apple"? I should have a right to transfer my account to "login with google/facebook/my own server" and it should work. The same way I can call my friend when he transfers his phone number to another phone network.

Trump banned from Twitter? He should be able to transfer his account to Parler and I should still see his tweets in my Twitter feed, be able to retweet it to my followers, interact with it, etc. Twitter wouldn't be able to influence tweets from Parler so it would show in timeline. But they could decide to not display it in proprietary parts of their system (search, trending topics, whatever)

This way we would prevent echo chambers and deplatforming, while simultaneously allowing companies to maintain their freedom to moderate their own space.