> Unfortunately, we take backwards compatibility very seriously in the core team and this is a big downside of this proposal.

Current implementation relies on that:
1. called is almost never used in practice (people just use .assert*)
2. The is True / False is discouraged and is rarely used by itself, let alone in combination with .called

> Wouldn't that also break any mock that is mocking an object with a "called_event" attribute?

It should break them in the same way as "called" breaks them now.