As you may know, the Microsoft Teams client is self-updating itself on a regular basis (documented as every 2 weeks).
That being said, it may be possible (and obviously it seems to be the case) that the Teams client is not being updated for a long period of time.
To ensure end-users are always running with the most up to date version, if the client did not update between 1 and 3 months they will start seeing a notification banner (in-app notification) while if it did not update for more than 3 months they will be asked to either update now or join the web experience.
As there is no reporting available to tell which Teams version a user is running you can check this script available in the Technet gallery to get the installed version https://gallery.technet.microsoft.com/MS-Teams-Get-installed-60c0dc5c