Yes, you can be removed from Steam for modifying Skyrim with Nexus Mods.
Yes, you can be removed from Steam for modifying Skyrim with Nexus Mods.
You've been playing Nexus Mods with Fallout: NV on Steam for more than two years and haven't faced any bans yet. It seems the restrictions you're seeing are likely limited to multiplayer features, not impacting your overall experience.
Using NMM hides your mods completely, and even with Steam Skyrim it remains a solo experience. If it supports mods, players can do whatever they like with it, and it would change things if it were modifiable. With multiplayer features, the situation is slightly different. Skyrim doesn't include MP.