Which multiplayer should we use?

So, Unity is going to remove to old uNet System by 2019.3, if I want to make a multiplayer game now, should I use the new system ( I think available through GitHub ) or should I use the old uNet System?

Depends when you are trying to ship and how much experience you have.

If you are new it’s best to just use the old one or even better Photon. The new system is just far too early to put into a production cycle, i’d estimate 1.5-2 years before it’s truly ready and not a struggle.

Once the new system is further along then it will be great. For now, it’s much simpler to stick with the old and learn as much ECS + Job system stuff you can along the way. The old system will be under Unity’s long term support so it is a sustainable option.