How to recover a podcast that has disappeared

I would guess that you don't have access to the old feed now? You've put the new feed's URL in the <itunes:new-feed-url> tag in the new feed. Did you put this tag in the old feed? If you didn't, it won't do any good in the new feed. If you can still edit the old feed you should add the tag, but since the Apple Podcasts page has gone dead I assume that feed is offline. IN this even you presumably wouldn't have access to the server to add a 301 redirect.

This lweaves only changing the feed URL in Podcasts Connect, which you've done. Doing this can take a couple of days to have an effect; however the danger is that if the original feed has gone dead that may have caused your podcast's removal from the Apple Podcasts lists. Also note that this method will not redirect subscribers on a Mac, or iTunes on Windows, as they read the feed directly.

As you've had an error message that suggests something has fallen over at Apple's end. Your only option, other than keeping trying, is to contact Podcasts Support - not at the URL you quoted which is for people subscribing so that they can make paywalled podcasts, but at https://podcastsconnect.apple.com/contact-us

For reference, the full processes for moving a feed URL is outlined at https://rfwilmut.net/notes/podcast.html#move