Onenote for Mac - Change layout back to old style - Tab bar missing!!!

I too prefer the older version.

This new version adds an extra step as I. Have to scroll back all the time.

how do I return to the original version?

Though, from an informational point of view, it's actually a very improved design.  

Assume you have 30 sections in a notebook.  You should know (with your degree and a small amount of knowledge of UI) that you shouldn't line up items (especially with variable widths) side by side, especially when they're important.  A horizontal list is not ideal.

This does not make any sense to me. You say you shouldn't line up times side by side, yet give no reason why not (the handy waving "it's obvious if you know knew more" is not a reason). There's a reason that browsers for example use tabs. You can give  a lot more information in a lot less space.

Personally this is a big step back for me, and if they'd wanted to unify the design in would have been much better to move other devices towards having tabs at the top. However I don't really know why desktop should be hamstrung by the abilities of a phone or tablet (by the way, the interface is still different on my phone so it's no a complete unification, they are more similar now though I'll admit). I'd rather have a similar UI designed to make the most of each device's strengths rather than a one size fits all approach. I wonder where the pressure for this feature was coming from, possibly this is easier for the developers.

Anyway, add me to the list of people who think this was a pretty terrible decision.

Oh yes, I'm sorry I forgot to mention that when you line up variable width items side by side, you eventually lose information to the right side of the screen.  It's not a primary action for a user to have to remember how to scroll sideways.  If you truncate the data to allow more tabs to line up, then you lose the information you placed there in the first place. 

Here's a quote from a Nielson Norman Group Article on tabs: "If you need more tabs than will fit in a single row, you must simplify your design."  One note followed most of the other rules on tabs, but this is an important one.  

"There's a reason that browsers for example use tabs. You can give  a lot more information in a lot less space."

"if they'd wanted to unify the design in would have been much better to move other devices towards having tabs at the top."

First of all, your first statement is not necessarily true.  The way One Note is doing it now, you can see a good 30-60 sections, whereas the way they did it with tabs, You could only see 10-15.  In the second quote, you're contradicting yourself here. Mobile browsers now have "slides" that you need to sort through.  They do not use tabs.  Sure it would be unified if they did that, but even more of a hassle to see all of the information.  

"However I don't really know why desktop should be hamstrung by the abilities of a phone or tablet"

I don't see why this is hamstringing anything.  You know you can resize the bar? You can even hide it if you wanted to giving you even more real estate.  Here's a couple of screen shots.  The first is on a 4k monitor.  There's no issue here obviously.  The second is on a 1920x1080 monitor which is turned 90 degrees for development.  I still don't see an issue and that's only 1080px wide.

Side Note: "GUI tabs are modeled after traditional card tabs inserted in paper files or card indexes" - Wiki. I don't necessarily agree with the physical-to-electronic user interfaces always being best.  Sure, they're great for older users who want something to feel familiar with, but they're not great for future proofing.  

I understand change is hard, the first thing I said in my post was that I didn't appreciate the change agreeing with what you're saying.  I had to relearn how to interact with the program.  I was frustrated at first too, but I did a little research and my own testing and found this to be the better design overall.  Microsoft did the same thing.  This isn't the days of  Vincent Connare anymore.  They have teams that do a lot of testing on this sort of thing.