Starting mi-August, the synchronization option available on SharePoint Online library will be updated to use the new OneDrive sync client.

Currently the ‘Sync’ button is still pointing to the previous OneDrive for Business sync client.

If your users are still using the old sync client at this time, the next time they will be upgraded to the latest sync client.

In addition of this change the administration option in SharePoint Online administration portal to define which sync client will be removed.