I've been afflicted by the inconsistent sorting order for a few months - chat shows the most recent message at the bottom, channel posts show the most recent message at the top. I'm using Teams version 1.6.00.26154 on a Mac, and 1.6.00.26163 on a Windows PC.
A couple of weeks ago, on both platforms I started to see a button asking if I wanted to try 'new Teams'. I've turned it on - it made no difference to the problem, and 'new' Teams doesn't include an option to control the sort order.
So I don't think the 'feature' is related to a version of Teams we can toggle on or off. I seem to recall that it was implemented automatically as part of a background update.