Procedural foot placement isn't a thing Valve invented, all the gigantic AAA games have it, and its now out of the box in UE5 with all their free animation systems. In terms of tech Alyx didn't do anything new, but it used proven techniques in a modern way to their maximum effectiveness. The baked lighting is the best in the industry (partially because every other game just dropped baked lighting entirely because its too much work and they can just offload the cost onto the end user with raytracing now), the actual design tools are the best in the industry, Valve knows that a level designer is not a 3d artist and vice versa, and as a result Hammer 2 has actual tools for level design and quick iteration and turnaround built in, instead of requiring you to do level design in a 3d modelling tool like every other engine (It also popularized trim sheet
hotspot texturing, which while not new, was not used for automatic one click whole scene texturing before Alyx).
I think the only new tech that Alyx pushed is the raytraced Steam Audio, with realtime and baked raytraced reverb, pathing, occlusion, and transmission simulation of sound waves based on the game scene. It has existed since around 2016 but for Alyx they battle tested it and added the audio pathing simulation specifically for the Jeff chapter.