Yes, I bought both content passes for Minecraft Dungeons, the expansion pass for Outer Worlds and I bought a Forza Horizon 4 ultimate upgrade code for £20. I would have for Sunset Overdrive too but the complete edition was like £9 at the time or something, it was cheaper to buy that than the DLC separate/expansion pass. I'll never purchase a standard edition game for £70 even if its a game I really want, so if I get said game through Game Pass then dropping £10-£20 on DLC for it is fine by me, I have Game Pass Ultimate until April 2024 anyway so its not as if it runs out anytime soon.
Would you buy DLC for a timed third party game pass game though? A game which you can't know how long it will be on the service for? Like Desperados III or Injustice 2 or packs for Assetto Corsa?