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.

@SmiggyG I'm assuming that it was a planned replacement for the One-Time Password Storage that is on the JPN server, but it has not actually been implemented.