In rare cases, the mailbox archive status seems stuck at “pending”:

If the status doesn’t change within some minutes after synchronizing Azure AD Connect, the user’s AD archive attributes may be “out-of-sync” with the Exchange Online configuration.
To fix this and re-align your AD properties you must first enable the archive to ensure the AD configuration matches your Exchange Online configuration:
- Select the Enable archive checkbox
- Click OK
- Select Synchronize AD with Office 365
- Click OK
Wait a few minutes for replication to complete before you re-open user properties and disable the mailbox archive:
- Deselect the Enable archive checkbox
- Click OK
- Select Synchronize AD with Office 365
- Click OK
After a few minutes the Mailbox archive status will change to Deprovisioned when you re-open user properties:

(if the archive status is stuck at “Provisioned (pending)” you can follow a similar strategy).