Any development on Unturned II was indefinitely paused 2 years ago (see: Unturned II Development Paused and One year later). Some of those planned features have made their way into Unturned instead, or are likely to make their way into the game in the future.
Nothing stopping you from making mods for it, it would probably require some sort of mod hosting website. It’s not at all on Nelson to set up steam workshop and modding support for a game he isn’t actively working on.
Maybe for games that are already popular. With a game as early in development as UII, I don’t think many people would attempt to add modding support to it.
I don’t see Nelson adding modding support to a game he isn’t developing, so I said that since that’s the only possible way mods could be added to Unturned 4.0