As it is now the word “standard” is doing a lot of heavy lifting. Google themselves aren’t even done with the specs so it’s not trivial to support all functions when they are not described. So there’s plenty of problems for 3rd party. A bit of info here.

One could argue that brake lights are a warning light 🤔.

Well… for now at least. Who’s to say that won’t be the next thing on their list. They do make a pretty penny on the play store as is, and could improve that if they banned side-loading. And let’s be honest, side-loading is probably a niche thing still.