It also has an option which lets you decide which folder the emails should be delivered to without the use on any Message Rules. Outlook can still be configured this way today without losing the indication by which account you received a message with.

In older versions of Outlook, all emails from all POP3 accounts used to be delivered to a single pst-file which created a Unified Inbox. When you configure multiple POP3 accounts in Outlook, all the emails will be delivered to their own dedicated pst-file.