URL transitions are rare precisely because it so hard to migrate people unless you have reliable redirects.
Choosing a host for me came down to one over-riding aspect: having my own domain for the feed. I hold little trust in the vague promise that the hosting providers will never go out of business and offer reasonable terms for hosting a 401 redirect for a year after switching providers. They hold the feed hostage and can wield almost all the power.
I think we are at a chicken-and-egg situation where very few hosts implement podcast:guid and practically no programs do anything with the tag. Feed generators on hosting platforms see no reason to support the tag because no apps support it, and no apps support the tag because no hosting platforms support it.
I see supporting this as an existential matter, not a nice-to-have matter. We can have an eco-system where RSS feeds are rare and only gigantic series can move because Namey McNameface and mcnameface.com are well-known quantities, where proprietary apps and the hosting platforms hold all the power, OR we see an eco-system of RSS feeds and imperfect but easier transitions.
Literally the only reason for a hosting platform to insist on holding onto all feed URLs while seemingly allowing own domains for podcasts is to hold the feeds hostage, to make migration less likely. (Looking at you, Transistor.FM.) This state of affairs ruins the ability of customers to shop around or to bargain.