Nexus mods shouldn't need Steam at all. Other than the steam_api.dll file, the file structure and all the files are identical, and all you have to do is out the mod files in the right place and turn them on in your load order. What mods do that? The only thing it really makes more difficult is if you want Steam Workshop mods. Which you can download, but you need to use a 3rd party tool to do it if you don't own the game on Steam.

There is also the new engine updates that fucked up old mods. Many Special Edition mods don't work on the Anniversary edition, and Fallout 4 was supposed to be getting similar treatment recently. I don't know if that is released yet or not, tho.