IINM, un-processed contents of encrypted messages do not constitute personally identifying information (PII), and if a user deleting their account or deleting a

IINM, un-processed contents of encrypted messages do not constitute personally identifying information (PII), and if a user deleting their account or deleting a message deletes the correlation between the message and their user/account/id from your servers. The complicated part is your intent to preserve it for the other participants in the thread. I think it's ok to allow a message author to cause their messages to be deleted from all other users as well (maybe leave a <deleted> bubble in its place). If an entire thread or account is deleted, you can just remove the association to the user and preserve the messages itself. It's a bit messy the more you want to preserve despite a user's wish to GDPR-bomb their data, but I don't think untenable.