We can all agree that the systems people have done for MMA over the years, are tried and proven in the ring, and their lineage is systems that were once taught for combat before firearms became implemented (this of course gets a little complicated culture to culture at a point in history, but you get the idea).
The fighting mechanics you learn in these systems apply broadly, and can be analogous to sort of your fighting operating system.
However--there are limitations to it. Your operating system is a platform, unequipped to preform some tasks by itself. There are certain things you're not gonna learn from the MMA competitive curriculum (because the only instance you're gonna scientifically test them, is out in the street, or the field), that others have personally experienced and devoted their career to codifying.
The list includes those instructors and their programs, and what their specialties are.
I'm not saying that this is a complete list, that they are the best, or that one HAS to learn from them--but they ARE learning resources for particular things, with references, a number of whom have trained together, and not some randos that decided to open up a Krav Maga school down the street.
Fairbarn was a street cop in Asia, dedicated his life to learning what was useful in Asian MA, and was brought in to teach WWII commandos and spies the basics of how to fight. As
@frank raud mentioned, Fairbarn Protocol is a good resource for this.
Carl Cestari was also a street cop, who dedicated his life similarly and learned from the surviving members of the WWII combatives program.
Kelly McCann is a Fairbarn and Applegate influenced instructor, who served in SOF and has been there, done that.
Lee Morrison is a life-long martial artist, who spent time in gangs and was involved in quite a few street altercations, and he and McCann have trained together.
Craig Douglas, one of the Shiv Works collective, was an undercover officer who's "been there, done that." One of his specialties is grappling over weapons in a car.
Libre Knife Fighting was founded by Scott Babb. He's a lifelong martial artist, who's dealt with a variety of knife-related altercations in his life, and developed a system that focuses more on how criminals use the knife. To my knowledge, he has trained with Mexican and Indonesian forces, and his students have helped him evolve the system, bringing back feedback from their real-life experiences on the street and field.
Piper was founded by Nigel Febuary and Llyod de Jongh, and it's the codification of how SA gangs and hoods are known to use knives.
Raul Martinez (Rogue Methods) was an infantryman and undercover cop, similar to Craig Douglas.
Ed Calderon was Mexican SF, who saw a lot of ****. He learned a lot of the people on the list, and has his own life experiences to teach from. His specialty is more situational awareness, travel safety, and counter-abduction.
Another name I should have added, is Brian Halliday, another MMA guy and life-long martial artist, who was in a biker gang and managed to survive an ungodly amount of street fights. He also helped develop Mastro Defense System.
The team at Arcadia Cognetari developed the marine corps Hunter program, which teaches spotting the pre-indicators of violent situations before they happen. Yousef Badou was an instructor for that program.
Given what I've seen on this website, it seems y'all know the rest.