Could someone shed some light on what's the legality of publishing emulated abandonware? I remember not too long ago Half-Life 2's web port got posted, is that still online?

Unless they've gotten a license to do it, it's a crystal clear copyright violation, generally done under the assumption/hope that an entity that has abandoned selling the game has also abandoned caring about enforcing its copyrights against such projects.

It's not legal. And in any case it's not abandonware, they're still actively selling these games (maybe not these exact versions, but I doubt a court would see that as a distinction with a difference).

Crystal clear illegal. And AFAIK Rockstar can be litigious.