It’s already included with Dota 2, SteamVR and Half-Life: Alyx. CS2 will also get the tools. As for a full-fledged SDK: nobody knows, when and if it’s going to ever be available for licensing. As far as I know, Garry is the only person who’s been granted a full licensed SDK so far.

Hmm so no standalone source2 sdk.
Kind of disappointed as a valve fanboy for 20+ years... I dont wanna be forced to make my game in unity or unreal.

I don't have alyx, cs2 or s&box.
I want to start porting my source game I've been working on to source2 and cant be limited to using Dota 2 or steamvr as a in-game dev environment.
Is it even worth porting my game levels to hammer2 since I can't write ingame engine features? Keys, weapons, inventory, etc

Nobody knows how extensible CS2 VScript is going to be. Ideally, in 2023 you should be able to make a game without ever touching the C++ side (like with UE Blueprints, but even with Unreal, to this day some APIs are still left unexposed to Blueprints and you have to either expose them yourself, or rely on third-party plugins), so actually your best bet at this moment for a Source 2 game base is S&Box, if you’re into that. Otherwise, Unity and Unreal are your best friends.