Does it work with my Garmin, COROS or Apple Watch?

Yes, indirectly. If your runs land on Strava, Negative Split can analyse them. It reads Strava’s web pages, so the recording device never matters.

Do I need a Strava subscription?

No. It works on free accounts, and most of what it adds (training load, Fitness/Fatigue/Form, race predictions, the year recap) overlaps with what the subscription sells. Subscribers can run both side by side.

What happens when Strava changes its website?

Affected sections disappear rather than break the page, and updates restore them, typically within days. There is no login or token involved, so recovery never needs anything from you beyond the automatic extension update.