There is nothing modern or antique, there are programs that do the right things and program that do the wrong ones. Classic shell is definitely among the former. It's light customizzable, you can have the w9x, xp, vista, 7, 8 behavior, you can choose among a vast library of skins. you can still use the ugly stock start menu with a click of mouse. I really can't find a single defect on it. Modern UI created a big problem and CS fixed it, perfectly.
P.S. Whatever i suggest to keep an eye on this thread https://forums.mydigitallife.net/threads/add-gui-to-server-core-1-0v.76023/ At least in theory is easier to add what you need on a bare windows than remove the crap from a full bloated one.
It's getting easier than it was just one year ago, because the UUP files. . Server where usable for general use since the days of server 2003, after a bunch of tweaks Since server 2016 practically nothing is missing. The BT is stack is the only relevant omission, but it's easy to fix using the Toshiba stack. Speaking about Core/Acore/HyperCore. Yes there is a reason. A very welcome one.
No, not multiplatform "stuff". Multidevice GUIs sucks, which is a different matter Just an example: think to Opera12 (or Firefox with less extent) Opera used to run everywhere. You had the same experience starting from OS2 to Windows, Linux, Solaris, FreeBSD and whatever. That's multiplatform thing, and was/is perfectly fine. BUT At the same time Opera did run on devices other than PCs, from Psion organizers to smart and dumb phones provided with a 80x100 display, to 60" TVs. In that case the GUI didn't pretend to be the same as PC one, the GUI was adapted for such kind of devices to provide a good (but different) user experience there. When that lesson was aired, the MS designers where doing something else, and didn't learn anything