I love Xfce, but from 4.16 and onwards they will push Client Side Decorations, which I hate.
The way I see it there are really two places to move to:
Mate or LXqt. I vastly prefer the GTK aestetics over qt, but it is still preferable over CSDs. Also unlike MATE LXqt has a built in compositor as far as I know? Please correct me if I’m wrong.
Since gtk is pretty moving to CSDs I think you are in a dilemma here.
lxqt doesn’t have a window manager at all really. The default is to use openbox which does not have a compositor. picom is easy to setup though. As an alternative, many people use kwin as well. kwin does have a compositor built-in
I think the only one NOT doing it is Cinnamon.
…But you’re right. Unfortunately. The easy solution would be to switch to Plasma but I HATE it. So I guess I have to try to frankenstein a DE out of either LXqt or LXDE.
Edit: Compton for me is a no-go since it looks HORRIBLE in so many programs (Firefox menus, for example have huge artifacts around them because compton puts the shadow in the wrong place).
Just an FYI, you can configure picom to avoid shadows on pop-up menus etc.
(Oh, also, marco has a built-in software compositor.)
I am still more looking at running LXqt or LXDE with Xfwm as window manager.
gtk3-classic?
But how do you implement that with Windows clearly designed for CSEs?
My plan is to move to either LXqt with Xfwm or to Mate with Xfwm, I think. I know you can replace the Window manager on both tho LXqt is easier.
I understand your pain - but there is time yet before it is inflicted on us. 4.15 is not done yet, and 4.16 will be the ‘release’ version that messes everything up. Even then, an IgnorePkg entry in /etc/pacman.conf could hold it off for a while longer! 
On a more hopeful note, there is a possibility that someone will fork it from version 4.14 - I live in hope! Too bad I am not up to such a challenge myself… 
There is already a fork specifically for libxfce4ui and the panel.
I thought I’d heard that, but wasn’t sure enough to post it. I wonder if it’s something I can help with?
Now let’s hope someone feels up to maintaining it.
(I Must admit being a tinkerer I am still tempted of trying LXqt with Nemo and the Xfwm… )
Do you have any idea what the CSDs could do for the usability and appearance of Xfce? I haven’t really thought about it yet.
Well two things.
The main one is that a CSD solution per definition has less useability since you cannot fit as many options in an CSD menu. Gnome is the perfect example, but then they really don’t like you making choices so…
The other of course that unlike in a menu bar based window you cannot hide the menu, so it is impossible to make the footprint smaller. For example the Xfce terminal most people hide everything but the window frame, but it is very easy to show again. With CSD you cannot do that, the window border is always taking up a LOT of space.
ok, thanks
Unfortunately, the reverse is also true - you can’t make it bigger easily either! A distinct disadvantage when dealing with hi dpi screens. There are also severely restrictive choices of appearance. On top of that, you must figure what CSS items are required to be changed to alter anything. Oh - and it’s likely to be ugly (see Gnome). 
arghhh …
Install gtk3-classic and find out.
Worst case just re-install gtk3.