This is really really weird then, on stable I can get Netflix L1 HD playback completely ok with unlocked bootloader and Magisk installedWith an unlocked bootloader Netflix returns a -9 error code, while with the locked bootloader it works fine. Logcat shows that the error is:
MediaDrm-JNI: Illegal state exception: Failed to get provision request: General DRM error (-2000)
This seems to be because of the entry
OEMCrypto_GenerateDerivedKeys: ends! returns 8
Instead it should return 0
Edit: okay, since Android 8.1, something has changed and I can confirm that you're right. In Android 8.0 you could have an unlocked bootloader and yet still have Netflix with L1, now in current stable you can't.
This is weird, since when I root my Pixel 2, I get L1 playback just fine on Netflix... OnePlus probably screwed something up again..