It’s not entirely clear, but I think it’s the fact you can simply 301 redirect to your new feed on a different platform.
Seems a bit misguided, because unless you’ve been pointing to your own domain the whole time, you still depend on the previous platform to 1) offer this feature at all, 2) keep serving that 301 forever or your old subscribers will be lost. Do podcast apps permanently change the feed URL when they see a 301? Am I missing something?
The fact is that I don't think it is well specified.
My feed reader will update the URL if the ending feed has a self URL that points at itself. But I don't think that is very common. In theory a permanent redirect should also be a signal to update the URL.
Seems a bit misguided, because unless you’ve been pointing to your own domain the whole time, you still depend on the previous platform to 1) offer this feature at all, 2) keep serving that 301 forever or your old subscribers will be lost. Do podcast apps permanently change the feed URL when they see a 301? Am I missing something?