What about relays that forget content after a period of time? These would be very lightweight and easy to run. Most people don't even want their notes stored for eternity.
Twitter grows by 12TB per day. If there was a relay that stores 30 days only for each user, that would still be 30 x 12TB - too much for anyone to store.
Although a lot of that is probably bloated media files, which relays can farm out to other servers (still have to address scaling for those though)