I think the closest you can get is using a VPS like DigitalOcean where you pay $X for a server and there's no autoscaling to worry about. But even with those, if you go over the bandwidth limit (although with DO the bandwidth limit is a lot higher) you would be charged more.

The unfortunate reality is that hobby developers that just want to pay $20/month aren't the target audience for GCP etc. They don't really care if you're using them less for your personal hobby projects. They target large enterprises, and those large enterprises would have very little use for something like "cap my spend at $20".

Even as an AWS employee, I sometimes use non-AWS hosting providers for my own projects. Even outside of the billing situation, AWS is often too complicated for my use cases. It's just not targeted at me and my hobby development projects.

disclaimer: am AWS employee but the above is my own opinion and not official position of the company, etc etc.