Microsoft introduced the SharePoint app bar early in 2021. This allowed for quick access to important content, global navigation, and content relevant to users such as a news feed and recent files.

The SharePoint app bar cannot be disabled permanently. However, it can be suppressed with PowerShell until March 31, 2023. The extension of this window is to allow organizations to complete their planning and change management process to prepare for this new functionality
What do you have to do (admin)?
If you would like to temporarily disable the SharePoint app bar follow these steps.
- Download the latest version of SharePoint Online PowerShell.
- Connect to your tenant (see Connect-SPOService (Microsoft.Online.SharePoint.PowerShell) | Microsoft Learn)
- Then, run the following command with administrator privileges:
Set-SPOTemporarilyDisableAppBar $true
To enable the SharePoint app bar that was previously disabled, run the following command in PowerShell:
Set-SPOTemporarilyDisableAppBar $false
When will this happen?
Temporarily hide SharePoint app bar available until March 31, 2023
Reference: Message Center Announcement