Definitely not an OS specific concept.  Big.little configs are all over the place on all OSes and architectures these days.  With a variety of ways OSes are experimenting with scheduling for them (nevermind that Android and iOS have been doing this for a decade).

from scheduling policies such as users, cgroup membership, container membership, group membership, nice level, to QoS attributes as you mention here...

If a given OS has specific APIs we should expose them via the os or _thread modules as appropriate.  But unless multiple OSes congeal around common concepts, there isn't much we can do for abstraction.  Perhaps a generic way to supply a generic OS specific settings instance to threading.Thread for implementation defined configuration purposes before Thread.start() is called.