I was very into the HL mod scene back in the day (as a player) and chatted extensively on IRC with makers. I was focused on single player mods and Neil Manke was my fav HL1 modder. Unfortunately his HL2 mod never got released.
Yeah exactly, and the golden age of modding, at least for FPS games. In a few years the engine evolves to something so advanced that only professionals can work on them. I consider HL2 the last moddable engine by the commoners. After that you gotta be a professional or someone with the patience of a saint to mod an FPS engine.
And nowadays they mostly use UE and Unity anyway. Only some of the boomer shooters are moddable.