Don’t risk losing your Contacts and Calendar when using IMAP in Outlook 2013, 2016, 2019, or Office 365

I’m using an IMAP account in Outlook as part of Office 365 and I noticed that various folders now have “This computer only” behind them. These folders I indeed cannot find back on the server.

This includes my Calendar and Contacts folder as well which I really cannot afford to lose.

What is the proper way to back them up?