NewsBlur already uses Reddit’s authenticated API for subreddit feeds, even though the subscription URL still ends in .rss. I checked the server logs, and Reddit is currently rejecting API requests with 403 errors. NewsBlur is incorrectly displaying those failures as 404, which explains why the RSS URL can still work in your browser.
The earlier 429 errors indicate rate limiting. There isn’t a confirmed workaround yet, and using the API doesn’t guarantee continued access if Reddit changes its restrictions. You don’t need to change your subscription URLs.