The major carriers perhaps, but support among the MVNOs isn't universal. Number sharing support for smart watch usage is almost non-existent among the MVNOs in Australia.

Eg. ALDI (yes, the German supermarket chain run a MVNO in Australia), have been saying esim support in the future since 2021.

In the US, the overlap between people who would buy the latest iPhones and the people who use MVNOs I would suspect is practically non existent.

The MVNOs here mostly market older, cheaper iPhones.