As with EVERY new feature like this with no real benefit, it should ALWAYS be an option, not forced.
And therefore, it is an option, one that is even off by default. Your IT department has enabled it themselves or by applying a security template.
They can turn it off again via the Set-ExternalInOutlook Exchange PowerShell cmdlet.
As the whole intention of this option is to let your IT department force the External awareness across all users within the organization, there is no end-user option to disable it again.
However, you can trick the implementation and still get rid of it as an end-user by creating a custom From field column.
For step-by-step instructions see; Remove “External” tag from Outlook Message List - Robert 365
Note that the current version of Outlook for Microsoft 365 no longer puts the External tag directly in front of the sender's name but on the far-right side (left from the received date/time) instead.