https://wccftech.com/there-are-already-fallout-76-mods/

Official Fallout 76 mods won’t be available for quite a while yet. That’s because Bethesda intends to tie them to private servers, which won’t launch until late 2019 at the earliest. Here’s what Pete Hines said recently:

Mod support is going to be tied to work that we’re also doing on private servers, and letting people do private servers. Mods will be a thing that run on private servers.