Here are some sources that track games on Xbox Game Pass and Playstation Plus subscription services:

https://docs.google.com/spreadsheets/d/1kspw-4paT-eE5-mrCrc4R9tg70lH2ZTFrJOUmOtOytg/edit#gid=0

https://docs.google.com/spreadsheets/d/19RorxFhWc2lHocg4c9zrVssSwZq1u2nPcpTsAvzdJQw/edit#gid=1938605355

As I am building the database of my own for Stadia games, I have some experience to to Stadia store. As I understand there are some challenges concerning correctly linking to the user's current region, as the links can vary based on were you are located, I would like to share my experience with this store.

Stadia introduced click and play link based on the structure https://stadia.google.com/player/ + <id_of_the_game_in_stadia_store>

When a user click on the link, if he already bought the game (or subscribed to a subscription channel like Ubisoft+), the user will lunch the game. If the user have not bought the game, but have a Stadia account, it will be redirect to the page og the game in the Stadia Store, except the user is underaged, in thi case the Stadia Store will return that the element is not found. Finally if the user doens't have a Stadia account, the user will be redirected to the Stadia homepage to subscribe for free to the service (Subscription to Stadia Pro is optional and can be now easily avoid). Once the user subscribed to Stadia, he or she will be allowed to access to the store.

So this store could be very easily integrated in igdb addind some values for cloud gamers using Stadia

Unfortunately, I haven't the same experience about how things works on GeForce Now (that doesn't have a store but aggregate games working on GFN from different store), or Game Pass Ultimate for games that could be run on cloud, but as cloud gaming grow up, such entry could be very useful for users that want to play in the cloud

Regards