Hello @uwzis, https://github.com/ValveSoftware/steam-for-linux/issues/8081#issuecomment-954352996 notes that Steam is looking at /proc/mounts to generate the list of potential partitions that could be used to add a secondary Steam library folder and this basic test is to check if your regular unprivileged user is allowed to write an executable file to it. Trying and failing to write to /boot/EFI should be harmless.
This issue is already being tracked at #3414. Closing in favor of the older issue report.
The more you know
I am very concerned about this script. Why would Steam need to access my boot-loader's directory?? Steam is still closed source unfortunately and I and many others would like an answer.