Just to be clear, straight off the top I'm not suggesting that this is what happened! Or even that anything remotely like it happened. Just that it to my mind potentially explains that I otherwise feel is a strange decision from Strava.
A while back I was watching a video on YouTube in which was said words to the effect of "There is no way that Garmin is going to buy Strava, they can get anything they want from Strava just by asking!"? (I think the video was by DC Rainmaker, but for the life of me I can't find it now!) Well I was thinking about that today, and I came up with the following which I freely state again is a conspiracy theory! I have absolutely no evidence that any of this is true, but here's my thoughts anyway :-
Now Garmin clearly benefits from the existence of Strava, but if Strava went away then Garmin could create a very similar service on Garmin Connect. In fact, just after Strava became popular they attempted to compete with it in such a way before giving up on the idea because they'd left it too long and Strava already had the "market" sewn up. However Garmin only does benefit if Strava encourages people to buy Garmin hardware. (Obviously people could buy a competing product from the likes of Karoo, Wahoo, Polar etc but there's a decent chance that if someone wants a GPS device to use with Strava it's going to be a Garmin. Maybe even more than one product if people want a bike unit and a watch!) If Strava have an app which allows you to do everything that you'd do with a Garmin, but using the phone that you already have, then that's a problem for Garmin - they have to make their product sufficiently better than the App to make sufficient people buy one, and that's a tough sell as it'll potentially be more expensive than the phone you're competing with was!
So did Garmin say words to the effect of "Strava, ditch sensor support in your app … Or else!"? It explains why they suddenly announced that they had a major bug in their app affecting lots of people (without there seemingly being lots of complaints about this app that crashes a lot?), and why instead of reducing
which sensors they supported or making it a "beta feature, engage at own risk!" they simply ditched sensor support and without any announcement about when or even if it would be coming back?
Incidentally I was doing the maths recently and felt the Strava App + Strava Summit + QuadLock mount would be a potentially better option than buying e.g. a Garmin Edge 830, but only if there was sensor support in the Strava phone app! And I suppose that's precisely the sort of calculation Garmin wouldn't want people making?