Google has confirmed that if a Stadia Pro subscriber decides not to renew their membership, they'll lose access to the games and discounts offered to Pro users but they will be able to keep playing the games that they've purchased.
Here's Google's full statement on the matter to Android Police:
"While we know that the free tier of Stadia is widely anticipated, we aren’t announcing specific timing just yet, beyond Phil’s interview in Protocol. If a gamer opts not to renew their Stadia Pro membership, they will lose access to the games and discounts offered exclusively to Pro subscribers, but they will be able keep playing the games they purchased, and they will have access to the Stadia store to purchase additional games."
This means that Stadia Pro subscribers will essentially be moved to a Stadia Base subscription if they don't renew their Pro membership. Stadia Base users can buy games from the Stadia store but they don't get free games or discounts on purchasing titles like Pro subscribers.
Also of note is that if a Stadia Pro subscriber declines to renew their membership, their free games will remain with their account and will be available again if they renew their Pro subscription. This is important to keep in mind since the free games offered to Stadia Pro subscribers are only offered for free for a limited time.
One other tidbit of Stadia news that came out recently is that the free Stadia Base service is coming soon. In an interview with Protocol, Google's Phil Harrison said that the free version of Stadia will launch "over the next few months".
Stadia launched on November 19th, 2019, meaning that the three-month subscription that early Stadia adopters got with their Founders or Premiere Edition is nearly over. If you've signed up for Stadia, what do you think of it so far?