I successfully tested servertravel with the Steam Subsystem using seamlesstravel. All you need to do, is activate the seamless travel checkbox on the GameMode-Blueprint and configure a Transition Map in the project settings.
Downside: You can’t use Play-In-Editor with multiple clients anymore, since seamlesstravel is not supported when running in single process. Launching multiple instances of the game on the same computer won’t really work either, since Steam does not support that, so you are basically forced to test multiplayer with multiple PCs when using Steam subsystem :(.
Edit:
I guess there is the workaround of closing Steam and then launching multiple game instances, though…