Is it possible to switch Ubuntu back to base-2?

The only way to do this, would be to patch (or in some cases, un-patch) all of the code which displays sizes to the user, and recompile your own fork of Ubuntu which has all of the sizes displayed using base 8 math (which is what you are asking for, and not base 2). It is a very invasive change to make, and is not an Ubuntu specific thing, as upstream applications were already doing this. The Ubuntu policy on size display is merely to clarify the matter, and to provide an official stance.

There is no setting which will switch between GiB and GB in all applications. If that's what you're looking for, the only way you're going to get it is to maintain patches to everything, which does so; unless you can convince all the upstreams to use such a setting, and provide patches, which they accept.

I'm sorry if you're looking for a simpler solution, but it doesn't exist, and isn't likely to anytime soon.

It is possible for KDE apps.

Just change it in your system settings.

First, install systemsettings clicking this

then, run it

Then, choose Locale ▸ Other ▸ Byte Size Units

select the one you want.