Using Classic Outlook on Windows for personal email, calendar, and contact management
Have you tried installing the latest Teams v2.1 (link below). Since installing I now see the Teams Slider but only in the New Outlook.
The add-in does not appear in Full Client and I'm on the latest version.
https://staticsint.teams.cdn.office.net/production-windows-x64/23307.2300.2497.5845/MSTeams-x64.msix https://staticsint.teams.cdn.office.net/production-windows-x86/23307.2300.2497.5845/MSTeams-x86.msix
New Outlook