It is standard in flight simulation since decades. So, yes, it is missing here. Beside the option of "tweaking" which could solve a lot of problems in TSW2.
Where? Last night, Roadmap Stream When? Last Night, Roadmap Stream PC Only? No. They are looking at an all-platforms option powered by DTL.
IF DTG themselves provide the file hosting, then they can avoid the licensing issues which were a big part of the problem; easy enough for them not to accept "any RL livery" or "any livery from a TOC not in-game" (this last might raise some interesting questions about RR represented in TS but not TSW, like Norfolk Southern).
Sharing liveries from the livery designer is not going to get you that as it wasn’t created in the livery designer, it’s hidden in the game files.
It's also much less appealing than it used to be if one has Cathcart, because for whatever reason the mod that unlocks it populates GWE with Scotrail 314s!
Most games with a livery editor have allowed it for years it should be standard. My opinion is though that I'd rather have a vast improvement of the editor as it seriously lacks behind most other games with them. It's a million miles behind GT Sport which I created perfect replicas of real race car liveries and even far behind the Ride and Moto GP series which aren't great by any means but passable. With the TSW editor there's too much compromise over what you're trying to achieve and that should have taken priority.