I don't know how each relay implementation works, but I always thought it made sense to only store recent data, except for profile and other more "static" data.
But I would assume for paid services that could be different.
Discussion
All of them store the data at lest for now. If they wouldn’t store then this would not work.
Imagine this.. you wrote this 4 hours ago .. your relays got the message.. but they will only distribute it to those with open websocket connections querying it
If my mobile app was not open then how can I receive your message?.
So now I’m receiving it because I’m connecting now, I’m publishing a subscription to a relay that queries all events from certain point onwards
So for events kind 1 they need to shore .. but maybe for kind3 that is the contacts they don’t need to store older versions and only keep the latest recevied one