I've noticed there's a 200 item Game Pass Storage listed in the manual. Below all of the listed storage options it reads as follows
- Gain access to Game Pass Storage by joining Xbox Game Pass.
- If your Xbox Game Pass expires, you can no longer deposit items in Game Pass Storage. To start depositing items again, join Xbox Game Pass.
https://pso2.com/manual/menu/storage
I have Game Pass Ultimate and I do not have any extra storage or any such option.