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RSS is more for getting new articles rather than getting all of them. Putting all the articles in the feed would be inefficient if you want to get the latest ones. Most feed readers cache articles so they aren't going to be deleted immediately.
If it's something you really want, feedly stores all articles as long as someone else subscribed to it, but feedly is enshittifed so it's not that great.
If you set it up with a reader that can hold onto old articles you can use RSS for this, but you'll have to set up your own time machine if you need that backlog of articles now.
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)
Best solution IMO is a social media or group chat (depending on persistent history) relay. Mastodon and Misskey forks are an easy way to share RSS feeds and discussion. I prefer the interface and features over RSS bots in Lemmy, but if you were managing a very large public platform the division of labor in moderation might be an overall plus, with some downsides.
As far as I understand it, the website controls what and how content is shared through RSS. So, RSS is not considered a way to archive content.
ON my blog for example, I have yet to figure out how to share the full content of the posts through RSS (I'm not that much of geek and, well, I just can't figure out how to ask my website generator, Hugo, to do that). So, for the time being all what my readers have access to is the summary and I don't think they can force change that and access the whole post through RSS.
Curious to see if anyone as more info on your question.