Rollback netcode can be thought of as the ability to accept inputs from the past as well as the present, rewinding and resimulating time to both handle inconsistencies and mask delay.
It has three requirements:
- Deterministic game state
- Hardware that can execute the game state at 10x speed for 10 frames of rollback, or so forth
- Manual handling of all gameplay effects not suited to "rewinding"
#1 is not simple, but all fighting games (including Smash) already do this. This is how replays work. So really, this discussion is only about the other 2.
#2 is a a big one, huge. If you want to accept an input that was supposed to happen 6 frames ago, you have to be able to recalculate every part of those 6 frame--in 1 actual frame. So the hardware has to be capable of running the game X times faster than normal, where X is the total number of frames you want to be able to roll back. This is why putting rollback on an old game (running on CPUs 20 years more advanced) is far more feasible and requires less engineering work.
This is a great oversimplification, but Smash Ultimate runs at 60 frames per second. To be able to rollback an additional 7 frames, the CPU would need to be able to run the same game at 480 frames per second. This is a great feat for a modern game on modern hardware, and requires a lot of special engineering. For the humble Switch CPU, running a far more busy 8-player fighting game, it's a challenge that has simply not been done.
Even if you acheived that, it still leaves #3. The example I like to give is camera effects. You know all those zoom effects Ultimate does? Rolling back any sort of camera effect would be awful and nausiating. So all the zoom effects have to be re-tuned around your rollback delay, and even normal camera behavior would have to be rewritten to not jump around with the game state changes. Sound effects also have to get a massive amount of attention to be polished--cutting off and jumping into sound effects mid-frame is jarring and bad. Particles are often a problem, since particle systems often have non-deterministic components in their libraries.
This isn't even getting into the weird gameplay design decisions. The Mortal Kombat guys actually refuse to rollback game-ending hits as policy, due to how aggrivating that would be. This sleight of hand wouldn't work with Smash.
#3 is a ton of work; for the amount of content in Ultimate it would easily amount to several engineers working for at least 2 years. But even that is nothing compared to #2: making a 60fps Switch game run at 8x speed
on a Switch.
tl;dr - Asking the devs to put rollback in Smash Ultimate is like asking your mechanic to put an electric engine in your tricked-out 2006 Chevy Silverado. It's just, not how
any of that works--but if it's any consolation their future trucks
will be electric.
