As someone who has attempted to license a paid Half-Life 2 mod, there's a pre and post-"Hunt Down The Freeman" era. From my understanding working with other licensees, it was so bad that they no longer want anyone working with their IP.

Valve doesn't work with the mod community anymore; they've pivoted hard into only working with UGC creators, which they do not consider the same.

Free mods can continue to exist, but it has to be within their walled garden, and done in ways that they approve of, and they offer no support or guidance. Even for licensees who have blessed support, they no longer have a dedicated Source Engine contact and haven't for years, or even over a decade(?).

Anything that is non-Source is a non-starter.

Most professionals have moved on to working with other IP or developing their own. Frankly, I'm surprised anyone does anything with Valve's stuff anymore, or they're too unfamiliar with working with Valve that they don't understand that their era of absorbing mods and turning them into full-fledged in-house IP is over and they don't treat fans the same way they did in the early aughts. The staff has changed dramatically and Newell himself appears less engaged in the company all together.

Seemingly, you can't get anything done at Valve, or working with them now. There are isolated teams there that work on hardware or existing legacy IP that generates cash flow from UGC work, but that's all Valve does now.