Garmin prefers to use their own Ant+ standard. Some garmin's do have BT. But in the past that's just been to facilitate connection to a smart phone. Not to allow connections to sensors. Maybe some do now, I'm sure others will post if they do.
You have to look at each garmin device individually and look at it's specs. They don't typically spec the BT version though in the manual. So if its a must have, I'd call.
If you are getting a new smart phone, I'd consider getting one that has Ant+ as well as BT. Then you'll have more options for fitness devices, but much less choice in phones. Samsung offers the most phones with Ant+ and BT.
PS. you are aware that BT5 devices will be backward compatible to earlier versions. So if the feature doesn't strictly have to have BT5, then it will still function.