@Nukleid Technically you won't be able to claimed Pro games from previous months. So basically you need to be subscribed for that particular month to claim that month's Pro games.
We're not sure if Destiny 2 is considered as part of Pro monthly game, or it's a special case. Considering it was heavily advertised as flagship game, I would be surprised if it gets removed in December (especially since we didn't have a full month in Nov to claim the game). We'll just have to wait for official confirmation from Google.
I highly suspect that since Destiny 2 is a free to play title, it will remain free to play however the collection with all current DLC will leave the Pro subscription in December. Once Stadia base comes out, people should be able to try the vanilla game for free. Pretty clever way to get Founders users to get their friends to try the service to play Destiny 2 with them and if they have the connection for it probably put more money into the service by subbing or buying things.