Never encountered these, what were you listening to?

Really? Not me.

I've seen opportunities to subscribe to podcasts for $$, which irritated me, but that's it.

This can't be right.

Same here. Spotify claims it's the podcast producers' doing when ads are inserted this way. It still miffs me. One of the conditions of being a paid content provider for Spotify should be that you provide an ad-free experience for premium subscribers.

This gets to a question of what’s an ad. Take EDM DJs. Could they advertise an upcoming festival they’re playing at, even though the main purpose of the podcast is playing music?

Spotify allows it, I've contacted their support team about it. They claim that as long as the creator is the one inserting the ads that they allow it.