403 is “forbidden”, which should not be used for that purpose … and I’ve just experimentally verified that bsky.app will return 429 (rate limited) when you attempt to fetch too many urls too quickly (eg. to see which are returning 200 vs. 403 say via a shell-script loop, heh).
I could easily believe that NB is hitting rate limiting, but even just reducing the OPML to a single bsky feed that I’ve just manually verified returns 200 OK with rss does not seem to work. 