some ISPs make it difficult to use other DNS servers
There's DNS over HTTPS they can't viably block, so thankfully they get the short end of the stick here.
edit: Thinking of it, anyone knows if it's possible to use that for OS-wide DNS resolves, not just for the browser?
Quad 1/8/9 isn't optimal alternative (too much centralization if everyone uses those by default) but running your own is easy.