In gdevelop native mode, it shows that network extension is deprecated, but I don’t see a replacement.
Will there be a new system in the future?
On an unrelated notice, it would be great to see a peer to peer, or even a cs system as extension for Javascript in the future (the latter require some backend, maybe on node js)
Great! Good luck. 
We don’t know about the future of network extension… It’s deprecated and will not be updated any more for now, maybe maybe, maybe it will repalced… But nothing can be sure on this point.
Gdevelop being an open source software, maybe a contributor could change this…
I would just like to say it’s quite a useful feature and it would be great to always have something to create network games with. If it got removed and there was no replacement in the future, we would lose multiplayer ability except with custom cpp libraries, which can be more complicated to implement in each game project.
So like you mentioned if someone with skill and time is interested in taking a look into this in the future, please do.