Bethesda has, in fact, taken issue with OpenMW in the past [0].

Bethesda at least had a specific reason for going after OpenMW, though. They’re okay with it now under “certain conditions” that I’m sure the OpenMW devs aren’t going to argue with.

Their main ask was that the OpenMW devs don’t advertise OpenMW as a way to play Morrowind on Android (or other platforms besides PC).

And there’s an understanding that OpenMW’s “intention” isn’t solely to be a way to play Morrowind, but also to implement a generic RPG engine and editor that uses Morrowind’s gameplay and file structure. For someone to make their own game with, not using any of Morrowind’s assets. (I’m not sure if any such game exists, though)

[0]: https://wiki.openmw.org/index.php?title=Bethesda_Emails