.. does not seem to be among the new health data, you can log.
Unless Garmin can use the data for some reason, there's no reason for it to log the information. You can manually log that in Apple Health if you want. There are also smart BP cuffs that have apps that will monitor it and provide meaningful analysis.
Yes needs manual input, can't be hard to do. One common reason for people embarking on fitness programmes is after advice from doctors regarding their blood pressure.
My Withings watch had it and they have one programmer and their dog in the development team.
Using Samsung Health for it at the moment but it would be better if all was in one place.