The liberal use of elbows in multiple situations is part of what separates MMA from all other combat sports. They are, of course, illegal in boxing and international-rules kickboxing. Muay Thai employs standing elbows but obviously not on the ground. Sambo allows ground strikes but not elbows. No grappling sport allows its competitors to elbow each other during takedown attempts or on the mat.
Only MMA presents these manifold opportunities. It has taken decades, but elbows are finally becoming a much more prominent piece of every fighter’s arsenal, particularly on the feet. They have long been employed on the ground, but only in the recent past has their full potential come into view.