Well I may be the minority opinion here, but I'd prefer this not be merged into the code base. To me the linked YouTube video doesn't look that good. IMO the horse is moving too fast, and the quiet sound (it's normally used when you move slowly with the sneak key) is a little odd, too.
We need to be able to move faster if we want to cross the vast distances and not use travel map all the time.
We can move faster with the console. I can understand, though, wanting a more convenient key that can be pushed and released whenever you want a burst of speed. We could add that as a cheat/development key.
If the goal is more about having enhanced horseriding, I would personally personally prefer to see (as an optional mod) less of a speed boost, maybe even the "faster" speed being what is the normal speed now, with the horse head bobbing up and down as the horse leaves and connects with the ground, and a more "galloping" sound.
While cantering technically isn't part of classic, purists can ignore it by simply not pressing the sprint key while riding. This fits my definition of horizontal improvement that adds quality of life without substantially changing the base game experience.
Well it's ultimately your decision, but IMO this falls a little outside the bounds of a quality-of-life improvement.
Basically, I'm a little apprehensive about this because it's adding to PlayerMotor.cs, one of the base files, with a mod that (right now, at least) doesn't really appeal to me that much, and I don't want to have to worry about breaking it when I work with the player motor.
It would be great if we could somehow have gameplay mods that don't require merging into the main source, that could be laid on top and used that way, like mods in the later Elder Scrolls games, with responsibility for maintaining them being with the mod author or volunteers.
If everyone but me wants this merged in, I guess I'll just accept it, but I'd prefer we could somehow find some other solution for gameplay mods other than merging them into the source (other than QoL and bug fixes, changes to make things work as they were probably intended in classic). Maybe there is some standard out there for mod support on Unity we can use.
Anyway I don't want to be a buzzkill. I think it's great if we can get gameplay mods going. And a horse sprinting mod could be cool.