You think that the ISPs are really bothered about whether you use DOH or a VPN to get around government or court mandated site blocks?
The ISPs will do the bare minimum to block access to websites that costs them the least amount of money to do, usually by sink holing the DNS requests on their DNS servers. If you happen to have your computer set to use different DNS servers or using DOH in your browser and can get passed the blocks, so what? The ISPs don't care as they still get your monthly payment and they can show they have implemented the blocks if someone from the government comes to inspect. The ISPs won't do anything about people using DOH until the law says they have to do so, as it will cost them money to install more equipment with no gain to them to do it off their own backs.