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This depends if ephemeral would be "lost forever" or "I just don't want it to take space on my storage". The latter can be implemented with a cron script on SSB :) The architectural problem (depending on how you look...) with SSB is that it's not only a storage but actually an append-only log, a bit like a blockchain. There were a lot of architectural disputes back when SSB wanted to introduce a "delete message" feature (which would be really "please don't show this").