Comment by asjo

rachelbythebay has a service and a series of blog posts about the technical side of this, starting at https://rachelbythebay.com/w/2024/05/27/feed/

TL;DR: readers should not poll more often than once and hour, use ETag and If-Modified-Since to determine whether to download the full feed again.

Which items you have seen previously is something the feed reader keeps track of.