For use cases where this level of rigour is desired, it would be nice to have real separate metadata vs. convention. Doing this by convention is unreliable.
You can get a basic level of enforcement for free by turning on the "Semantic Pull Requests" bot that will let you know when you forget the type (or use an invalid one):
https://github.com/probot/semantic-pull-requests
It obviously won't catch your mistake if you forget to mark a breaking change as breaking, but it's a start.