Wait a minute! I use KDE Plasma for at least 2 years constantly and it knows my monitors. I use one at work and one at home and when I plug them it automatically changes resolution and keeps the wallpaper I set for each. Also, got a lot of colleagues switch to Plasma from Gnome for exactly this reason!
Does it keep your windows in place? Resize/move them properly when you switch to a smaller resolution screen then resize/move them back again when you go back to UHD?
I found the situation is worse if I extend my monitor's screen onto the laptop, because when I unplug the monitor and plug it back again, all hell breaks loose.
(This with the latest KDE that Ubuntu 19.10 has)
Mostly the same as Gnome, since eOS is Gnome based. If you're interested Egee on YouTube does a test called "Distro Delves" on many distros, Elementry included, and one of his tests he does on all of them is multi-monitor support.
Strange, because I use plasma (and XFCE before that) and apart some minor issue with scaling it works without problems on a 4k laptop monitor plus 2 external HiDpi monitors
Display port works better than HDMI though
KDE just gets total confused when I connect or disconnect a new display, windows jump all over the place, sometimes they move out of the visible screen area if I switch between hiDPI and non-hiDPI displays, or even just disconnect and then reconnect a UHD monitor. I've seen some reports pointing fingers at Qt as the main culprit, tbh I don't care, it doesn't work for the end user.
The latest Ubuntu on Gnome at least doesn't get confused, but it does not recognize my monitor sometimes (I have to turn it off then on again, for example when waking up from suspend), and switching between displays takes about 20-30 seconds (I have my laptop screen disabled if a monitor is connected). But it doesn't remember my open windows, nor the past position of my terminal, which is set to auto-exec on login. This is something that works much better in KDE.