The GUI of the Microsoft Purview compliance center doesn’t support the exclusion of selected mailboxes when the special All target is used. However, you can use PowerShell to add mailbox exclusions to sensitivity label policies, including adding the members of a group as exclusions.
A change in how Office apps apply mandatory labeling as dictated by sensitivity label policies means that both new and old documents are processed. New documents have always been dealt with; the change being made ensures that Office apps detect the lack of a label when opening an existing document and will apply mandatory labeling at that point. It’s a change to help customers move on from the unified labeling client.