You do need client support but these days that’s not uncommon: Android Pie, Chrome, Firefox, and curl all have built-in support and there are apps for iOS and Android, and once you’ve enabled it it will work almost everywhere. I believe most clients default to dual resolution so it won’t break if you’re on a network which interferes (e.g. a ton of Cisco captive portals used that address in error) unless you’ve enabled hard-fail mode.
Correct - third party software is needed (as far as I know) for DoH.