Babel doesn't have a spec per se. They try to be compliant with the ECMAScript spec, but implement popular proposals behind presets or plugins. You can read about their relationship to ECMAScript and the TC39 committee on their blog: https://babeljs.io/blog/2018/08/27/7.0.0#babels-role.

Getting something through the TC39 is a pretty lengthy process, you can read about it here https://tc39.es/process-document/.