Hmm. The good news is I got the original "News" section to reappear. First I removed the Global Client setting that enabled it, added it back and then re-enabled it. I also disabled all three of the Personal Sections I'd created from appearing, just to go back to the sidebar's original state. Then I deleted the Channels app (latest TestFlight) and reinstalled it. Upon launch, I now see the original "News" section in the General > Manage Navigation settings.
Unfortunately the bad news is, now when I try to re-enable any one of my three Personal Sections via the same Global Client Settings, the Channels client I'm looking at immediately crashes. I can't add any of them back like I had them before. The client app crashes immediately with every attempt. It's 100% reproducible. Subsequent launch attempts crash immediately too. I have to disable the Personal Section(s) first, and then and only then can I prevent the Channels app from crashing, on launch or otherwise.
I submitted logs just now, hopefully that will help?