This is what will probably happen.
-Game gets announced for the WS and M-A-Y-B-E for the Game Pass PC Lineup (if thats ready to launch)
-A year will pass and then THQ Nordic will announce for the Gamescom a Steam release with a physical edition for Europe.
i mean - the only reason why i think it might come to steam eventually is that it's a third-party game and Insomniac owns the IP (but not the game itself) ... so more or less the same situation as with all the other MS-funded games that eventually came to steam (unlike stuff like Forza or Halo which are made by MS-owned studios)
If it's third party funded game (aka Recore, Lucky's or Quantum Break) I just assume it will reach Steam sooner or later by THQ (all, I repeat, ALL those games got the same treatment 12 to 18 months later) so I don't see why Sunset Overdrive would be the exception, in fact, GamePass is not a big change right here because all the last releases on Steam were already on the service and that wasn't a problem for THQ to get an agreement with MS.
Timed exclusivity? maybe or yes. Full exclusivity? I don't expect that.