The change being made here is to switch sites using “legacy” (hamburger) over to the new “header dropdown” (hamburger) menu.
We aren’t forcing everyone to use the sidebar and header dropdown should work fine with the categories + latest home page option.
The user and site wide customizable sections you mentioned are certainly still evolving, but that is just new stuff.
As we start rolling things out more widely, sites will still have the opportunity to opt out and use the hamburger for some time. We’ll use that feedback (and feedback here) to help identify whether there are any other changes required before it’s final removal.
We don’t plan to do that until 3.1, which is likely to be sometime in the July/August timeframe.
I misspoke there. We will likely do this sooner if we don’t discover other issues along the way that need more time.