The Vi [0] headphones / AI running trainer had heart rate sensors. AFAIK they were optical though, similar to wrist devices.
[0] https://www.businessinsider.com/guides/tech/vi-sense-wireles...
Unclear if using microphones to track heartbeat also works when the AirPods are also playing music.
They know what they are pushing to the speakers, so couldn’t they negate the music to make that work?
The signal to noise might be really tough, but that would be neat!