(Not sure if this repo is still active but I'll file the issue just-in-case.)

Atom has support for marking entries deleted: https://datatracker.ietf.org/doc/html/draft-snell-atompub-tombstones-18

It would be useful if JSONFeed supported something similar. Without an explicit tombstone entry, deletion can only be inferred by fetching the entire feed, including following all next_url links.

This is also important for push services like WebSub, which currently have no way of notifying a subscriber of deleted items in a JSONFeed.