Not positive, but since they don't say for free users only I'm assuming it's everybody. Mainly for their own exclusive "podcasts" but available for other networks to use also. I get the impression these take the place of the traditional "host reads a sponsor message" time slot.

https://www.theverge.com/2020/1/8/21056336/spotify-streaming...

> Spotify Podcast Ads offer the intimacy and quality of traditional podcast ads with the precision and transparency of modern-day digital marketing

Yeah, that's what I like about online targeted advertising. The transparency.