Hey folks!
From a conceptual perspective, launchers do not fit the confinement model very well. If launched applications are going to use the launcher’s runtime, then the launcher would require every single permission bit, as it is not possible to know beforehand which permissions will be needed by all the applications that may be launched. This means that the launcher would need to be effectively unconfined.
Steam was a exceptional case that required some alignment, and according to [steam-support documentation](https://snapcraft.io/docs/steam-support-interface), “Only the Steam snap may establish this interface.” In general, -support interfaces are designed for specific applications and should not be granted in other cases. Thus, -1 from me for granting umu-run any use of steam-support interface (#voteAgainst).
Sorry for the inconvenience.