It's a really good question!
If one person on your server has followed the account, all the posts after that moment should be visible from your server. However, anything before that may not be.
The solution to this would be "backfilling", where your server automatically checks for older posts when someone follows an account (or perhaps just interacts with it?).
Backfilling can be very resource intensive, which is apparently why it hasn't been implemented yet, but...
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