To add onto this: you may need to remove some options on Linux for installing games and whatnot if you implement this, as there's no difference between the "desktop" and "start menu" for the portal.
Also noting that this wouldn't just help with a sandboxed Steam, this would make the standard Steam client fit in with the desktop well too.
Looking at how Epiphany (GNOME Web) implements it may be useful: https://gitlab.gnome.org/GNOME/epiphany/-/blob/6e5357947679e48aa1c043f221425255105937a1/lib/ephy-flatpak-utils.c#L137
I really hope this gets implemented soon, it's inconvenient to have to use workaround scripts or just launch games in the flatpaked Steam directly.
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Please consider to implement org.freedesktop.portal.DynamicLauncher to create menu shortcuts for games. This will help sandboxed Steam client.