The multiplayer mod for Just Cause 3 has been put into permanent standstill, with one of the core developers being recruited to work at Avalanche Studios.
Cameron Foote, the lead developer on the team working on the multiplayer mod for Just Cause 3, wrote that he was "approached with an offer I couldn't refuse". "As part of [Avalanche Studios, makers of Just Cause 3], I'll be able to bring many of the things you know and love from JC2-MP into future Avalanche experiences, while reaching heights that a mere mod couldn't facilitate," Foote wrote.
Unfortunately his hiring, as well as a lack of time on the part of the other contributors, means that development on a multiplayer mod for Just Cause 3 is being stopped indefinitely.
"We will still be maintaining JC2-MP where appropriate - while we're not actively adding new features, we will fix bugs, maintain stability, and ensure that JC2-MP remains a quality experience for players present and future."
Just Cause 3's game director Roland Lesterlin told me last year that Avalanche couldn't officially support a multiplayer mod, but they "certainly wouldn't be closed door" if modders wanted to make one. And they surely must have known one was in the works, given the success of the same creation for Just Cause 2.
The hiring means it's pretty likely that we'll see an official multiplayer version of JC3 in the future, as opposed to an unofficial one. The game should have a good resurgence in interest once that happens; hopefully it's not too long away. (I do wonder how nice the explosions will be on a GTX 1070/1080, though.)