I'll write more comments later, but now I'm in a hurry and just want to leave an important comment:

I think we should start supporting post messages that have the actual text content as a blob. Authors may want to soft delete that blob if they regret posting it. I make the distinction between soft delete and hard delete, where soft is when you agree to not replicate something anymore, while hard would be the absolute and instantaneous "right to be forgotten", which we know is impossible to guarantee in p2p. I think we should start work on allowing soft edits and soft deletes, and I'm considering that the mmmmm clients would have that from the beginning.

It doesn't pertain so much to this thread because gladly we're talking about other metadata fields, but since we're at it, we can reconsider the whole thread system.