Wouldn't a better alternative be to design a messenger that complies with GDPR? Simple user accounts that can be deleted at the request of the user, peer-to-peer encryption (and where possible, communication), a "storage cabinet" for each user where encrypted data end in when the user is offline (with an encryption/decryption key that is generated client-side and transmitted while both users communicate) and can easily be deleted and i think this covers most uses.

This is just an idea that i came up with right now, but if you start your design with the goal to store as little data as possible and anything you store needs to be both encrypted and easy to delete, then i believe you can come up with several ideas for most issues.

It also helps to see this as respecting the users' privacy and giving them control, as opposed to a development burden :-P.