Making a fighting game is not easy, for a variety of reasons that range from making them completely deterministic, to underestimating their complexity, to the amount of graphical assets needed, to the balancing and fine tuning required to produce something worth playing. So, it’s natural that we would all be grateful if there existed some engines that could ease the pain and allow for starting development with the smallest overhead possible.
(Un)suprisingly, there aren’t many modern engines that cover this particular niche. You have UFE2, a fairly complete fighting game engine that works with Unity and supports rollback netcode (but uses Photon as a middleware to connect players). Then, there is an Unreal Engine template for 2D fighting games in the works, another Unreal Engine template for anime fighting games, and maybe a platform fighter engine or two.
If we remove the requirement for a modern engine, most people familiar with fighting games know of one that is still pretty popular, despite being relatively dated: the one and only M.U.G.E.N., originally developed by the now defunct company Elecbyte. Being an old engine, made by a company which disappeared from the radar and with no clear copyright holder, is a recipe for disaster: M.U.G.E.N. games cannot legally be sold, which means that they are relegated to non-commercial projects, handily crafted fan games or broken messes with several hundreds characters.