This is a problem for a handful of people in our organization (exchange).
I didn't have the issue until today, but all of a sudden it has replicated to me. In Outlook, if you click File > Info > Manage Add-Ins, previously there were five (iirc) Add-Ins present for every user that were enabled by default, and had a description somewhere
along the lines of "This extension is provided by default from Microsoft and cannot be uninstalled". The list is now blank, and attempting to add those default add-ins back from the store is not possible, as they don't exist as downloadable options.
Can confirm that this issue just cropped up a few days ago at most.
EDIT: For what it's worth, I'm on the semi-annual channel and on build 1808 currently. The other users affected are on the monthly channel and are on the newest build (1904). As soon as this problem occurred on my local laptop, I went to check on another
machine I use remotely (that I haven't touched outlook on in months) and the problem was there as well. I have a suspicion that this is Office 365 credential-based.