Try enabling Simple List Extensions[1] on your RSS feed and see if that deletes the old items. Basically this tells the feed reader that your feed isn't a typical "stream of posts", it's a finite list. So it should be treated differently.

Newsgator used to handle that by deleting any posts not in your feed at every retrieval. So maybe Feedly would too.

If you think that guessing at the internal implementation of Feedly's code based on what a company that's been dead for years used to do sounds like a long shot, then bravo. But hey, it's worth a try, right?

It's been a minute, but I think the gist of it is adding

  xmlns:cf="https://www.microsoft.com/schemas/rss/core/2005"

to your <rss> root element, and

  <cf:treatAs>list</cf:treatAs>

under the <channel>. But see the link for examples. Good luck! [1] - https://docs.microsoft.com/en-us/previous-versions/windows/d...