Whoa, weird!
Turns out you actually can upload here, you just can’t embed the upload… but if you paste the image into the text box, wait for it to finish, and then remove the exclamation mark from the beginning, it works.
If the original upload is:
Just remove the ! at the very beginning:
[Uploaded image in the Pluribus thread - About This Message Board - Straight Dope Message Board - December 29 @ 10-23-35 AM@2x|690x293](upload://a2dZuVuXpN6mKF9ecOtdOzG8oZd.jpeg)
And then it works…
Did it always work like that, or was this a recent change/bug? How bizarre!
Maybe it’s related to this Discourse issue: Users not in Embedded media post allowed groups can bypass upload restriction by copying the uploaded image link - Bug - Discourse Meta