

It’s free for a limited number of queries per month. Generally enough for one device (depending on personal usage, obviously). Edit: It’ll still function as DNS after that limit, you just won’t get ad blocking and whatnot.
Mullvad has a free ad-blocking DNS you can use, too. I recommend the “base” version: https://mullvad.net/en/help/dns-over-https-and-dns-over-tls

Well crud. Thanks for the heads-up. First I’ve heard.
Sounds like Quad9 doesn’t actually do ad blocking, so that’s not much of a replacement.