No, OToon only support URP and plan to focus on URP only.
OToon tested in URP version that bound with unity 2019, 2020’s LTS version, which is URP 7.X & URP10.X. Addition test in URP 12 is also workable but since unity 2021 is not in LTS now and consider not stable enough, I recommend you install unity 2019/2020 and install URP via package manager if possible.
Currently, Otoon is not support GPU Instancing.
Yes, Otoon shader is SRP Batcher compatible.
It means your project have not install URP correctly, re-install the recommend version of URP in the package manager.
Close all your IDE(vscode, visual studio, rider…etc). Then, reimport the whole project.
Make sure folder Assets/OToon- URP Toon Shading/Shaders/Library exist and has otoon files inside, Close all your IDE(vscode, visual studio, rider…etc). Then, reimport the whole project.