@bender@twtxt.net 404 could be indeed a temporary error if the file resides on a mounted remote filesystem and then the mount point fails for some reason. With a symlink from the web root to the file on the mount, the web server probably will not recognize the mount point failure as such. Thus, it might not reply with a 503 Service Unavailable (or something like that), but 404 Not Found instead. (I could be wrong on that, though.)
The right™ way is to signal 410 Gone if the feed does not exist anymore and will not come back to life again. But that’s hard to come by in the wild. Somebody has to manually configure that in almost all situations.
But yes, as @falsifian@www.falsifian.org points out, exponential backoff looks like a good strategy. Probably even report a failure to users somehow, so they can check and potentially unsubscribe.