as you sensed, it’s under the website control: readers just check the rss ‘‘file’’ and show it to you in a nice way, every now and then they check for new articles and if your readers supports it you can save the older ones as they come

there can be also limits hardcoded in the reader that don’t let you take more than x items from a new feed but anyway the most majority of rss files just have the last y items, it’s rare that they expose everything (it would be a mess)

Mastodon also got the rss for user’s media, useful to follow artists