Show HN: history4feed – get a historic, full text archive for any RSS feed
github.comAuthor here with a disclaimer, it's not perfect, but works well with popular blogs more likely to be indexed by WBM.
I am sure there are more stable/accurate ways to do this with scraping but the WBM approach suited my requirements.
See some example usage of history4feed here: https://www.dogesec.com/blog/full_text_rss_atom_blog_feeds/