Hello,
(I am new to KDE and looking for the "best distro for KDE" I installed KDE Neon (as I was OK with the Ubuntu LTS package base).)
For long, long time I use (left) Alt + Shift to change my keyboard layouts. Now I am unsure that I can do it in KDE.
I found the place to change it, I see the default is Ctrl+Alt+k, but I want to use Alt+Shift.
I also see there are a lot of keyboard shortcuts registered with Alt+Shift+[some other key(s)] by deault, so I started setting them to None, as I did not need them:
In "Global Shortcuts":
ksmserver:
- "Halt Without Confirmation" (default: Ctlr+Alt+Shift+PageDown)
- "Log Out Without Confirmation" (default: Ctrl+Alt+Shift+Del)
- "Reboot Without Confirmation" (default: Ctlr+Alt+Shift+PageUp)
KWin:
- "Walk Through Windows (Reverse)" (default: Alt+Shift+Backtab)
- "Suspend Compositing" (default: Alt+Shift+F12) - BTW what a usefult default shortcut, like "shoot myself in my foot"
"Run Command":
- "Run Command on clipboard contents" (default: Alt+Shift+F2)

In "Standard Shortcuts"
- Back: (default: "Alt+Shift+Left")
- Forward: (default: "Alt+Shift+Right")

So I set these all to None, but it is still not possible to assign the keyboard changing shortcut to just Alt+Shift (without any more keys). Come on, I could do it without any problems in XFCE and in Cinnamon, is it some way to configure KDE to accept it?
(I found another Alt+Shift topic here (without any answer), but I thought it was cleaner to start a new one.)

I have given up , I set it to Alt + Shift + a, which is similar to my original shortcut. Bad, bad Plasma.

There are a couple of other threads which discuss using Meta by itself as a shortcut to open the application launcher, which included descriptions of using lower-level x key binding settings to allow you to do that, and later ksuperkey was introduced as a more end-user-friendly solution (but you still needed to install it separately).

Now in KDE 5 (I don't remember which version, but it was in one of the earlier releases), they made some changes to allow binding those modifiers by themselves, but I don't know off-hand how far they went with that (I know Meta is supported, probably Alt as well).

Thank you so much for the answer. What confused me when setting the shortcut for switching the keys was that when running systemsettings5 (go to Keyboard -> Layouts), somehow "Alternative Shortcut" was set and I tried to re-set it to Alt + Shift, which it did not allow (for some reason). Somehow I ignored the two other options: "Main Shortcuts" and "3rd level shortcuts" just above it. IMHO it is quite overcomplicated. Anyway, I managed to set my needed configuration by setting the "Main Shortcuts" to Alt + Shift, and setting the "Alternative shortcut" to "None" ("3rd level shortcuts" was and is also "None").