The base game (Destiny2) is only playable for Pro Players. I have a Pro subscription.
When I buy an expansion of Destiny2 (Forsaken), what happens when my Pro subscription expires?
Can I play the expansion (Forsaken) on Stadia Base?
Can I play the base game (Destiny2) on Stadia Base?
In this case it looks like Destiny 2 is already within the Bundle "Forsaken", therefore I believe you are basically buying Destiny 2 (base game) + the add-on (Forsaken) with that price, so even if your Stadia Pro runs out in this case, I believe you're still be able to play Destiny 2 then.
This is how it looks in the Stadia Store:
In this case it looks like Destiny 2 is already within the Bundle "Forsaken", therefore I believe you are basically buying Destiny 2 (base game) + the add-on (Forsaken) with that price, so even if your Stadia Pro runs out in this case, I believe you're still be able to play Destiny 2 then.
This is how it looks in the Stadia Store: