Is that more the case for particular categories of podcasts? Because I haven't noticed this myself. I mainly listen to stand-up comedian/general comedy podcasts and the usual advertising suspects are Raid Shadow Legends, NordVPN, etc. Other than recurring guests/hosts, the podcasts I watch seem mostly disjoint from one another.

What sort of post-apocalyptic hellscape did you find these podcasts in?

I mostly listen to fiction podcasts, and other than mentioning related podcasts/magazines in author/narrator bios, they don't really sponsor each other. (Some do have sponsors; all have patreons, merchandise, etc.)

Are you hitting a Spotify "coordinated destruction of all that is good and holy" effect, or is it some genre-specific thing (like drop shipping cryptobro podcasts or something)?

I'm honestly curious, despite being snarky.

it's especially annoying when they start to advertise each podcast on each other's feed and you get a sort of quadratic spam across every channel.

I do a podcast.

A lot of that is a tried and true method of promotion for those coming into podcasting without a big following elsewhere to try to convert.

We started with a fairly large social media following that we could promote with, but others without such an existing audience are better off advertising on similar shows. After all, that's the draw of podcasting as a medium... advertising efficacy is pretty high.