This was never an intended way to detect the container environment, and in particular is not provided by non-pressure-vessel containers and sandboxes such as Flatpak. We should remove it before people start relying on it. The intended interface is that they should look for /run/host/container-manager, as per https://systemd.io/CONTAINER_INTERFACE/.
We added this in commit 7460b0cd "pv-wrap: Temporarily provide /run/host/container-runtime" as a hotfix workaround for a SDL bug, since updating SDL would have been a riskier change to make to the stable branch without going through a beta.
Removing this will cause controller hotplug detection to regress in games that bundled their own copy of SDL between 2.25.x (sometime around September 2022) and 2.26.4. This regression can be avoided by not bundling SDL, and instead relying on the Steam Runtime to provide a sufficiently recent version.
This reverts commit 7460b0cd.
I think now that SDL 2.26.3 (with the container detection fix backported) is in the stable release of soldier, it's time for us to revert this in the next beta.