I wouldn't begin to claim I have a solution, but I think there's steps Twitter can take to show they are acting in good faith. I believe often times, Twitter flags accounts for suspicious activity - for accounts to be reactivated, a phone number has to be provided. If Twitter wanted to, are they not capable of deploying an algorithm that serves as a dragnet to catch accounts that tick off multiple check-boxes for bot activity and flag them? This would not mean that the account would immediately be banned or shadow-banned, but they could provide some kind of visual indicator to other users that the account in question has recently been flagged for bot-like activity (on the account's comments, retweets, etc.).

There are many real humans who have no phone number.

Or would never give it to Twitter.