Understand: Licensing
Licensing is a subscription-based model per user.
How to count required licenses
To calculate the number of licenses you need, count the number of users within your Microsoft 365 with a subscription. You do not need to count the same user more than once across multiple Microsoft 365 services (e.g., the same Exchange Online, SharePoint Online and OneDrive for Business. A user = one backup license).
If you have a departing employee, licenses are not adjusted in the same moment as the user/license is changed. Depending on the time of your backup job versus when the time of day licenses are counted, because of this, it may take 2-4 days for the user to leave the backup. Moreover, if you have multiple applications, the backup must recognize it in both Teams, Exchange, OneDrive, and SharePoint.
From here the user will be found in ITP (Insider Threat Protection) for 30 days, and then it will be archived.