Microsoft today released a new feature to Teams that lets you customize the app's left sidebar. This has been confirmed through a response by a Teams engineering representative to a post on Team's user feedback forum.
The new capability allows you to select which apps you would like to be pinned to the left sidebar. This lets you access your frequently used tools more quickly.
To get started, simply click on the overflow menu indicated by three horizontal dots on the left side of Teams and then right-click on any app available. Once that is done, you will see an option to pin that app. You can also search for apps you want to pin. Doing this will add the app you've chosen to the group of apps attached to the left navigation bar. You can also drag the items to change their arrangement in any way you want.
For messaging, you can also pin apps beside the formatting options in your messages. To do that, just click on the overflow menu below the message box and then right-click on any app. Of course, you can unpin any of your pinned apps anytime. You can refer to Microsoft's guide to learn more about the new feature.