Set flight signing values for bootmgr
Enrolling in the WIP sets the flightsigning value not only for the OS partition, but also for the boot manager, so this should be set/unset when enrolling/un-enrolling.
Is it safe for multi-boot systems? or it would probably affect all Windows OSs?
Safe in which way? It's not going to break another Windows installation. Microsoft itself sets this value, if you have an enrolled machine at hand, you could verify this.
If it's working with {current}, why the change?
I think it would be relevant for when a new Insider build also installs a new boot manager.