To me podcasts that aren't publicly availability aren't podcasts. And I don't mean that in a technical definition way, I think they are just a different - way less interesting - medium.
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To be more precise: Spotify (Audible/Apple/etc) exclusive podcasts are podcasts in the same way that a Facebook feed is a blog.
@map
To me it's not a podcast if there's no RSS feed available. I don't want to have to through some crappy platform or app. I don't mind if it's "premium" in the sense that the feed may be protected by standard methods, special URLs etc.
I have already forgone interesting sounding "podcasts" for that very reason.