tried: dnf downgrade plasma-workspace ... which resulted in /usr/bin/startkde returning,
but that didn't work; ksmserver died with an unresolved symbol.

running startkde from failsafe didn't work either, and neither did my workaround 
of running kwin and plasmashell by hand.

So re-upgraded; tried using kdm's 'plasma' session type; this resulted
in just a background fc31 image but no menus or any functionality; so 
I'm back to my workaround mentioned originally again.

The latest plasma release did indeed change that to '/usr/bin/startplasma-x11', try that name instead

That said, logging in graphically should work too, no idea why you're only getting wallpaper.

I've modified my .Xclients to use /usr/bin/startplasma-x11  and that seems to work.

I'm puzzled as to why such a well known command would be suddenly renamed though, probably causing
lots of issues with others also.

BTW, xorg-x11-xinit-1.4.0-5.fc31.x86_64

still mentions startkde in /etc/X11/xinit/Xsession

Not sure about the wallpaper thing; plasma and kwin were both running; just not doing much.  I'll debug
that further...

I created a symlink ( /usr/bin/startkde -> /usr/bin/startplasma-x11 ) and everything seems to work correctly after that. That may be the easiest approach to fixing it long term. Updating the xorg-x11-xinit package to replace startkde with startplasma-x11 would also work, but there may be other places that 'startkde' is expected to exist.

Nominating as a prioritized bug because this could potentially affect everyone who runs KDE.

%changelog
* Mon Dec 23 2019 Rex Dieter <rdieter> - 5.17.4-2
- provide compat /usr/bin/startkde symlink (#1785826, #1785973)

FEDORA-2019-499b84d03f has been submitted as an update to Fedora 31. https://bodhi.fedoraproject.org/updates/FEDORA-2019-499b84d03f

This bug causes tigervnc-server to fail, as ~/.vnc/xstartup has,
exec /etc/X11/xinit/xinitrc

which has,
elif [ -f /etc/X11/xinit/Xclients ]; then

which has,
STARTKDE="$(type -p startkde)"

I am using KDE spin, so only KDE desktop loaded.
I note, rpms switchdesk, switchdesk-gui, directly reference 'startkde' in created user files, ~/.Xclients-default, ~/.Xclients-default

The soft link work around fixes the problem for me.

*** Bug 1785858 has been marked as a duplicate of this bug. ***

plasma-workspace-5.17.4-2.fc31 has been pushed to the Fedora 31 testing repository. If problems still persist, please make note of it in this bug report.
See https://fedoraproject.org/wiki/QA:Updates_Testing for
instructions on how to install test updates.
You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-2019-499b84d03f

bluedevil-5.17.5-1.fc31, breeze-gtk-5.17.5-1.fc31, grub2-breeze-theme-5.17.5-1.fc31, kactivitymanagerd-5.17.5-1.fc31, kde-cli-tools-5.17.5-1.fc31, kde-gtk-config-5.17.5-1.fc31, kdecoration-5.17.5-1.fc31, kdeplasma-addons-5.17.5-1.fc31, kgamma-5.17.5-1.fc31, khotkeys-5.17.5-1.fc31, kinfocenter-5.17.5-1.fc31, kmenuedit-5.17.5-1.fc31, kscreen-5.17.5-1.fc31, kscreenlocker-5.17.5-1.fc31, ksshaskpass-5.17.5-1.fc31, ksysguard-5.17.5-1.fc31, kwayland-integration-5.17.5-1.fc31, kwin-5.17.5-1.fc31, kwrited-5.17.5-1.fc31, libkscreen-qt5-5.17.5-1.fc31, libksysguard-5.17.5-1.fc31, pam-kwallet-5.17.5-1.fc31, plasma-breeze-5.17.5-1.fc31, plasma-browser-integration-5.17.5-1.fc31, plasma-desktop-5.17.5-1.fc31, plasma-discover-5.17.5-1.fc31, plasma-drkonqi-5.17.5-1.fc31, plasma-integration-5.17.5-1.fc31, plasma-milou-5.17.5-1.fc31, plasma-nm-5.17.5-1.fc31, plasma-oxygen-5.17.5-1.fc31, plasma-pa-5.17.5-1.fc31, plasma-sdk-5.17.5-1.fc31, plasma-systemsettings-5.17.5-1.fc31, plasma-user-manager-5.17.5-1.fc31, plasma-vault-5.17.5-1.fc31, plasma-workspace-5.17.5-1.fc31, plasma-workspace-wallpapers-5.17.5-1.fc31, plymouth-kcm-5.17.5-1.fc31, plymouth-theme-breeze-5.17.5-1.fc31, polkit-kde-5.17.5-1.fc31, powerdevil-5.17.5-1.fc31, sddm-kcm-5.17.5-1.fc31, xdg-desktop-portal-kde-5.17.5-1.fc31 has been pushed to the Fedora 31 testing repository. If problems still persist, please make note of it in this bug report.
See https://fedoraproject.org/wiki/QA:Updates_Testing for
instructions on how to install test updates.
You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-2020-515c3c4b9d

Adding that `switchdesk` should reference `/usr/bin/startplasma-x11` instead of '/usr/bin/startkde'. See `/usr/share/switchdesk/Xclients.kde`. This was already mentioned in a prior comment #7.

bluedevil-5.17.5-1.fc31, breeze-gtk-5.17.5-1.fc31, grub2-breeze-theme-5.17.5-1.fc31, kactivitymanagerd-5.17.5-1.fc31, kde-cli-tools-5.17.5-1.fc31, kde-gtk-config-5.17.5-1.fc31, kdecoration-5.17.5-1.fc31, kdeplasma-addons-5.17.5-1.fc31, kgamma-5.17.5-1.fc31, khotkeys-5.17.5-1.fc31, kinfocenter-5.17.5-1.fc31, kmenuedit-5.17.5-1.fc31, kscreen-5.17.5-1.fc31, kscreenlocker-5.17.5-1.fc31, ksshaskpass-5.17.5-1.fc31, ksysguard-5.17.5-1.fc31, kwayland-integration-5.17.5-1.fc31, kwin-5.17.5-1.fc31, kwrited-5.17.5-1.fc31, libkscreen-qt5-5.17.5-1.fc31, libksysguard-5.17.5-1.fc31, pam-kwallet-5.17.5-1.fc31, plasma-breeze-5.17.5-1.fc31, plasma-browser-integration-5.17.5-1.fc31, plasma-desktop-5.17.5-1.fc31, plasma-discover-5.17.5-1.fc31, plasma-drkonqi-5.17.5-1.fc31, plasma-integration-5.17.5-1.fc31, plasma-milou-5.17.5-1.fc31, plasma-nm-5.17.5-1.fc31, plasma-oxygen-5.17.5-1.fc31, plasma-pa-5.17.5-1.fc31, plasma-sdk-5.17.5-1.fc31, plasma-systemsettings-5.17.5-1.fc31, plasma-user-manager-5.17.5-1.fc31, plasma-vault-5.17.5-1.fc31, plasma-workspace-5.17.5-1.fc31, plasma-workspace-wallpapers-5.17.5-1.fc31, plymouth-kcm-5.17.5-1.fc31, plymouth-theme-breeze-5.17.5-1.fc31, polkit-kde-5.17.5-1.fc31, powerdevil-5.17.5-1.fc31, sddm-kcm-5.17.5-1.fc31, xdg-desktop-portal-kde-5.17.5-1.fc31 has been pushed to the Fedora 31 stable repository. If problems still persist, please make note of it in this bug report.