Glad this exists! Just come across it while searching for something else and put it on my iPhone. I have a Pi Hole on my local network but this is great for when I'm using mobile data. I've configured my Android phones to use AdGuard DNS in the past via DoT and I'm glad this option is now available on iOS too even if it's a little more fiddly to setup.
For those like me who have a Pi Hole at home and don't need this active all the time, you can disable it at any time in your settings. It's listed under the VPN settings, a new option pops up called "profiles", and there you can set DNS to "automatic" which will pick up your WiFi's DNS server through DHCP like normal.