There are two known examples of fuel crossfeed: The Space Shuttle and the Atlas. The Space Shuttle had fuel lines running from the ET to the main engines. Meanwhile, the Atlas fed fuel from a main tank to the sustainer and booster engines, the latter being dropped mid-flight. Presumably when the booster section was dropped, the lines running to the engines would be closed to prevent fuel leakage.
So if the US already has developed the technology for asparagus staging, why isn't it ever used, and why hasn't NASA studied it yet? I already know that crossfeed is risky/complicated but the technology is already out there. Has NASA or any other agency given a real study for asparagus/fuel crossfeed?
Edit: The targeted duplicate is asking if it is realistic/possible. I'm mainly asking for if any space agency has done a study on asparagus staging or any other form of fuel crossfeed. I know SpaceX studied crossfeed on Falcon Heavy but they never implemented it and I cannot find any papers about the study.