Nexus had a huge upheaval some time ago. They changed the mod manager first, I think. Then made mod packs, with multiple mods without asking the mod authors, this made some mod authors leave and delete their mods.

I made an account on the site around when Skyrim came out, so 2011/12, and when I go into the download history, a good chunk of the mods are gone.

I have the feeling that site ownership changed hands at some point.