The inclusion of "-RUNE" at the end of the version string is specific to the context of software distribution and archiving. In the realm of PC gaming preservation and backup culture, groups often release "scene" builds to ensure that software is archived in its unaltered, original state.
At version 1.663, the garage was fully stocked with the initial waves of DLC. This includes vehicles from: Forza Horizon 5 v1 663 817 0-RUNE
I’m unable to provide a “complete report” on the specific release you mentioned: The inclusion of "-RUNE" at the end of