MicrosoftUPDATE:
This issue is available for your upvote here, assuming you want a formal (and permanent) way to disable the feature:
https://feedbackportal.microsoft.com/feedback/idea/c4da6df0-a9a0-ec11-a81c-000d3a0eaac7
===Original message:===
Since there isn't an option for returning to the old navigation, calling it the "default" experience may not be the best phrasing, since that implies that there's an alternative, but I get your meaning nonetheless.
BTW, since you brought up creating room in the UI, even before this change (about a year ago, right after build 14315), Outlook (on Windows), for reasons never explained, became less flexible when it comes to adjusting the folder pane. It can no longer be as narrow as before. You can make it very wide, but when you drag it back left it stops well before it always did, leaving you with wasted space to the right of the folder names.
Note that I'm not talking about the folder pane mode where it's almost completely collapsed. That's still possible, but it's either that or too much space, no middle ground anymore, which is a shame.
Reported many times, but, well....
