Do I need a subscription to get useful data from a fitness tracker?

It depends on the brand. Fitbit's Inspire 3 ships with a 6-month Premium trial, and consumerreports.org notes that some advanced insights sit behind Premium — but reddit r/fitbit users report the core sleep, heart-rate and workout data is usable without paying. WHOOP is different: the device is bundled with a mandatory 12-month membership, so there is effectively no 'no-subscription' mode. Garmin, Amazfit and Xiaomi require no subscription for their core features.