You misunderstand. I'm not advocating running a local web-server. That's bad. What I'm saying is that none of the big actors are using custom protocols anymor

You misunderstand. I'm not advocating running a local web-server. That's bad. What I'm saying is that none of the big actors are using custom protocols anymore even though lots of them used to, because browser security policies have rendered them useless for regular end-user interaction. They have all found other approaches instead. (Some bad, like here. Others better, like the solutions I chose instead).