Hilariously, we have the need for the opposite... SEC requires record keeping of written communication. HIPAA covered industries face a similar plight. There is no great chat solution for this... RocketChat is a mess, it used mongodb as a backend and the server side requires roughly 512mb of RAM per user. Mattermost is not much better, it also uses a terrible database choice and has similar scalability issues, and the license is a minefield. Both have major usability issues. Both crash a lot.

I do appreciate E2E encryption for some narrow use cases, but it is not the end-all-be-all.