Fitbit OS on the Sense and Versa 3 won't be available on earlier wearables
Sunday, September 27, 2020 at 9:24PM 
Fitbit Sense (Source: Fitbit)
Even if the Fitbit Versa 2 is just a year old, unfortunately, it won't be getting the latest version of Fitbit's smartwatch software. The Fitbit OS 5.0 platform was developed "exclusively" for the newer wearables and "optimized" for the Sense and Versa 3's latest displays and speaker technology. OS 5.0 introduces a new interface navigation method and interface that takes advantage of the higher resolution screens. And while it can run older apps, these need to run in a compatibility mode.
Developers can't distribute a single version of an app optimized for both old and new smartwatches. What they can do is release different versions that will reach the right users. Those who own older Fitbit Versa smartwatches can still use theirs just fine (our first-generation Versa still works well); it's just they won't get the steady stream of new features.
Source: Engadget