Q:

SPFx version in SharePoint 2019 On Premise

Anyone knows which version om SPFx will be available in SharePoint Server 2019 on premise? 

A:

SharePoint Server 2019 will ship with SPFx version 1.4.1. 
Vesa Juvonen, Senior Project Manager for SharePoint, mentioned this in one of the events a few days back.  
Since SP 2019 will have inbuilt support for modern team(not connected to O365 groups though) and communication sites, the SPFx extensions will also work here.