Yeah, every now and then you have to go through and delete old saves. There's definitely a cap that causes problems, especially because there's no warning that you're hitting the cap.
That said, I just had an odd experience earlier where I lost the ability to save my game. No reason, just the save option was greyed out so I couldn't click on it. I made sure I had plenty of space left, which I did, and I even deleted as many old saves as possible. Still, I had the problem where I couldn't save anything, no matter how many times I reloaded.
It wasn't until I closed out of Stadia completely and came back later that things cleared up. I'm not sure what the problem was, but it's working again for me now.