https://www.pcgamer.com/fallout-76-mods-are-definitely-coming-but-may-not-arrive-until-late-2019/

Everything we know about Fallout 76's private servers suggests they'll arrive sometime after launch, they'll be online-only, and, yes, they'll support mods. According to Pete Hines, though, we may have to wait a while for the latter. 

In conversation with Gamespot, the Bethesda VP suggested mod support might not surface until November 2019 at the earliest.