I sure wish that option was available for me. I made the enormous mistake of opting in to New Teams a few months ago, and have never been able to go back to seeing new channel posts on the bottom. Of course I flipped the "Try new Teams" toggle offand it made no difference at all. Literally not one thing in the program reverted. I uninstalled Teams (including doing a "thorough" uninstall recommended by an MS support, which had me clear Appdata folders and such), and then installed Teams again from a new download, but post order is still most recent on top, even though I had the Toggle at the top clearly showing New Teams is off.
I saw MossPine 's post, and was hopeful, but that option does not exist for me. I just flipped the toggle for New Teams on again, and literally the only thing it changed is putting a "New" label on the icon. All channel posts still have newest on top, and still no option for me to change the order. Does that toggle button actually work for other people? Turning New Teams on or off does not change one single pixel in any part of the program, as far as I can tell.
Honestly it wouldn't infuriate me so much if they just made conversations in Chats and Channels consistent, instead of having to scroll up to view history in one and scroll down to view history in the other. Put newest on top, or on the bottom, but why have it different in different parts of the program?
I used to be a defender of Teams before this UI change. Now it drives me insane about 100 times per day and I honestly just can't recommend it for a business that wants something consistent and reliable.