[averagemodder]

In practice, Bethesda does not permit mods with assets from a different Bethesda game. Everything else is negotiable, there are kid toys trains turned into Skyrim dragons, but mods with assets from another Bethesda game (or even from a DLC of the same game, but without making the mod dependent on that DLC) are taken down.

So, modding projects which plan to accomplish that with some installers extracting those assets tend to operate a bit under the radar, legal gray area or not.