For some reason Garmin has chosen not to back-port the respiration rate feature into watches like the 935/Fenix 5 series, even when you use their (expensive) supported HRM strap like HRM-TRI
They even own firstbeat now so licensing cost isn't the issue, they just don't want to bother even with porting the widget.
But the data of course is still there if you have HRV turned on and the appropriate FIT field has to be there since Garmin uses it.
I was thinking if clever developers can make Garmin apps like "HRV Test" then it should be possible to look at HRV data in a FIT file and find the R-to-R peaks to calculate respiration rate?
If it helps anyone, this is Garmin's explainer on respiration and supported devices/straps: