It doesn't work without a chest strap.

My VivoActive 4 tracks this without any external accessories. I've never really looked at the data and can't tell you how accurate it is though.

I have spent some time testing it, it is useless. Maybe it's accurate if you're not doing controlled breathing. I haven't had someone else watch me and compare. (I am also a trained vocalist and do a fair amount of yoga, it might be that it's only accurate for people with minimal breath training, or people with more exercise-focused breath training.)

I have a Garmin 245 that reports respiration without a chest strap. It tracks throughout the day. You can view on a line chart, or view awake/asleep averages. No idea how it measures, or if it’s accurate.

I've been testing this with and without the chest strap and it's not at all correct. I deliberately slow my breathing to 1-2 breaths per minute and it goes from saying 13 to 12. The metric is there, I don't trust it at all.