Fitbit OS 5.0 has been launched for the Fitbit Sense and Versa 3. It has been developed exclusively for these smartwatches and will not be released for the older models. The update includes changes in UI and navigational updates.

source https://gadgets.ndtv.com/wearables/news/fitbit-os-5-0-launch-exclusive-sense-versa-3-ui-navigational-updates-2302093#rss-gadgets-all