Searching 'no ads' seemingly won't query that tag, seemingly.
I'm a little surprised you can't search for "no ads" apps outside of e.g. Pass listings, but I'm really glad they're not defaulting to just a title text search and cluttered listings of Farming Simulator [NO ADS] [NO IAPS] [NO REFERRALS] [NO TIMEWAITS] [NO DLC].
there is no economic interest from google to make ad free apps actually findable, and no chance somebody making a better play store is going to usurp their position.
I'm not even sure I'm convinced that ad-free apps don't generate revenue for Google, even ignoring the ad-free apps that DO generate revenue for Google through IAPs and other non-ad means.
The fact that they don't have any support for filtering just says to me that they don't really care about app discovery outside of what they build suggested lists for, which seems orthogonal here since some of the suggested lists they curate are literally titled "Ad-free games" (plus "Offline games", "Premium games", "No-interruption games", etc which are also all largely filled with no-ad games, with a few exceptions).
Maybe they're omitting entire features / filter systems just so they don't have to add the sub-ability to just see which apps don't have ads, but I doubt it. As other comments have stated, a lot of people just don't want Amazon-like product titles crammed with keywords on keywords, especially when they're not even checked for correctness. I'm one of those people.
I'd say this probably boils down to a differing fundamental assumption we have: your comment seems to imply you might think big companies are inherently out to maximize profits through manipulating people, even if that means intentionally making a product worse for customers. If that's the case, there's really no point in debating here since that's a completely different conversation and I doubt either of us have the necessary internal evidence needed to convince the other of the real intentions behind withholding this specific feature.