Backfeed’s service is degraded until services at archive.org return to normal. They are recovering from a security incident. See here for updates.
Backfeed takes an RSS feed and concatenates it with deduplicated historical items from snapshots of the feed saved in the archive.org Wayback Machine. You’ll get a Backfeed URL to enter into your news aggregator or podcatcher which will give you access to a larger set of items than the official feed. This is useful when a RSS feed author includes only a few items in the latest feed, but you know there are older items which are just not listed.
https://backfeed.app/KEY/URL
where KEY
is your access key and URL
is the feed’s original URL. Full API docs here.