I'm not a developer of RR. So, I can't say if its developer will prepare the ROM sources for the latest Android versions. I'm just building RR for Mi Max and implementing the latest security patches. The latest RR sources update was few months ago.
Moreover, I'm not quite good for preparing device sources for the newer Android versions. So, I have to say that Android 10 is my final station.
As for the L1 Widevine... I'm not good with it as well. And can't say if it's possible to get it or not. As far as I know, L1 refers to a hardware DRM. Isn't it?