Can't tell whether default HD is serverside now, but it would make sense if it was. If anyone really wanted to implement a serverside HD in a custom system that was better than what I had implemented, you'd need to be able to test an area for players (serverside) for a specified period of time, spawn an NPC for every player, or essentially multiplex my original method across NPCs.

Flashback to my solution: When a player used their weapon, a single global NPC would essentially warp to the coordinates on a schedule and test if any player was in a hitbox. There was no way at the time to specify a duration for this kind of thing, which is probably still the case.

I don't know about the possibility of having an NPC spawn for every player, but when there is an upper limit to the number of people allowed to use weapons in a level, you could potentially accomplish this by adding NPCs to each level (e.g. 2 for every 1v1 spar room) and having them use the trigger that moves them and potentially playertouchsme events.

Multiplexing my original solution would be the same thing, but with a fixed number of global NPCs attempting to fill the time gaps depending on demand (i.e. lots of overlapping weapons usage is less reliable than if there were fewer people using weapons at the same time than there were global NPCs to handle HD).

Edit: I never moved recoil/blink off of Master Storm's systems (I assume Thor did), so if you're talking about that I don't have anything to offer. I pretty much reverse engineered everything else movement-related from the default systems, and I can't imagine Thor didn't take advantage of looking at my code when redesigning the movement system, potentially leaving recoil as something no one ever reverse engineered.