The Toolbar options that were at the bottom of the Outlook composer have been integrated with the top Toolbar. So the Toolbar is not missing at the bottom, it has been removed and placed at the top.

To add a button, click the "Customize Quick Access Toolbar" down arrow, and either choose one of the options available in ...

Now click on the three dots (More option) and select the “Show history button in toolbar” option as shown in the above picture. Similarly, if you want to remove or hide the history button from ...