KDE Settings -> "Default Applications" does not exist anymore in 24.04 LTS.

[deemon]

After upgrading Kubuntu 22.04 LTS to 24.04 LTS my KDE settings has lost "Default Applications" settings.

[claydoh]

What Global theme, color scheme, icon set are you using?
I only ask because the non-highlighted text "diagnostics" is yellow, which isn't right.

That might be nothing, though. You definitely are missing something.

First, make sure you are actually fully up tom date

sudo apt update

sudo apt full-upgrade

and to make sure all Kubuntu components are actually set to installed, if certain meta-packages were removed due to certain default components were uninstalled in the past.:

sudo apt install kubuntu-desktop

post any errors

next, try resuming any package installs that were interrupted or incomplete, and then install any missing packages.

sudo dpkg --configure -a

sudo apt -f install

If errors, post them.

If the above doesn't work, then try running one of the specific System Settings modules to see if it puts out any errors, which may point to what is missing.

kcmshell5 kcm_desktoppaths

and even try running System Settings from the terminal as well, which may also provide clues if it puts out any errors.

systemsettings5

[deemon]

Thank you very much. Apparently I broke something via trying to remove seemingly unnecessary "oxygen" style... because using "breeze". ( `sudo apt autoremove --purge kde-style-oxygen-qt5 kwin-decoration-oxygen oxygen-sounds liboxygenstyleconfig5-5 liboxygenstyle5-5` )
And it kind of caused very bad sideeffects. Especially bad when I did reboot and desktop was gone entirely... `sudo apt install kde-plasma-desktop` and relogin fixed it though. And "default applications" also came back.

I only ask because the non-highlighted text "diagnostics" is yellow, which isn't right.

because of annotation... I did annotate a yellow marker circle there :-D

sudo apt install kubuntu-desktop

oh noes. although it would have probably fixed it also, it would have been really bad idea and counterproductive, as I just tried to de-bloat kubuntu and it would have put 99% of it back.

$ sudo apt install kubuntu-desktop
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
The following additional packages will be installed:
bluez-cups cups cups-browsed cups-bsd cups-client cups-common cups-core-drivers cups-daemon cups-filters cups-filters-core-drivers cups-ipp-utils cups-pk-helper cups-ppdc cups-server-common dc
espeak-ng-data gir1.2-atspi-2.0 gir1.2-gstreamer-1.0 gir1.2-notify-0.7 gir1.2-secret-1 gir1.2-wnck-3.0 hplip hplip-data kwin-decoration-oxygen libao-common libao4 libbrlapi0.8 libcupsfilters2-common
libcupsfilters2t64 libcupsimage2t64 libdotconf0 libespeak-ng1 libgutenprint-common libgutenprint9 libhpmud0 libimagequant0 libkf5sane-data libkf5sane5 libksanecore1 liblept5 liblouis-data liblouis20
liblouisutdml-bin liblouisutdml-data liblouisutdml9t64 libpcaudio0 libpoppler-cpp0t64 libppd2 libppd2-common libqpdf29t64 libraqm0 libsane-hpaio libsonic0 libstartup-notification0 libtesseract5
libuniconf4.6t64 libwnck-3-0 libwnck-3-common libwvstreams4.6t64-base libwvstreams4.6t64-extras mscompress openprinting-ppds orca plasma-theme-oxygen print-manager printer-driver-brlaser
printer-driver-c2esp printer-driver-foo2zjs printer-driver-foo2zjs-common printer-driver-gutenprint printer-driver-hpcups printer-driver-m2300w printer-driver-min12xxw printer-driver-pnm2ppa
printer-driver-postscript-hp printer-driver-ptouch printer-driver-pxljr printer-driver-sag-gdi printer-driver-splix python3-brlapi python3-cups python3-cupshelpers python3-louis python3-olefile
python3-pexpect python3-pil python3-ptyprocess python3-speechd skanlite skanpage sound-icons speech-dispatcher speech-dispatcher-audio-plugins speech-dispatcher-espeak-ng system-config-printer-common
system-config-printer-udev wvdial xbrlapi
Suggested packages:
cups-pdf inetutils-inetd | inet-superserver antiword docx2txt lynx hplip-doc hplip-gui python3-notify2 python3-reportlab system-config-printer libaudio2 libsndio6.1 gutenprint-locales ooo2dbk rtf2xml
hpijs-ppds brltty psutils hannah-foo2zjs tix tk gutenprint-doc magicfilter | apsfilter python-pexpect-doc python-pil-doc libttspico-utils espeak mbrola speech-dispatcher-doc-cs speech-dispatcher-festival
speech-dispatcher-cicero speech-dispatcher-flite speech-dispatcher-espeak gnome-software python3-smbc
Recommended packages:
printer-driver-braille
The following NEW packages will be installed
bluez-cups cups cups-browsed cups-bsd cups-client cups-common cups-core-drivers cups-daemon cups-filters cups-filters-core-drivers cups-ipp-utils cups-pk-helper cups-ppdc cups-server-common dc
espeak-ng-data gir1.2-atspi-2.0 gir1.2-gstreamer-1.0 gir1.2-notify-0.7 gir1.2-secret-1 gir1.2-wnck-3.0 hplip hplip-data kubuntu-desktop kwin-decoration-oxygen libao-common libao4 libbrlapi0.8
libcupsfilters2-common libcupsfilters2t64 libcupsimage2t64 libdotconf0 libespeak-ng1 libgutenprint-common libgutenprint9 libhpmud0 libimagequant0 libkf5sane-data libkf5sane5 libksanecore1 liblept5
liblouis-data liblouis20 liblouisutdml-bin liblouisutdml-data liblouisutdml9t64 libpcaudio0 libpoppler-cpp0t64 libppd2 libppd2-common libqpdf29t64 libraqm0 libsane-hpaio libsonic0 libstartup-notification0
libtesseract5 libuniconf4.6t64 libwnck-3-0 libwnck-3-common libwvstreams4.6t64-base libwvstreams4.6t64-extras mscompress openprinting-ppds orca plasma-theme-oxygen print-manager printer-driver-brlaser
printer-driver-c2esp printer-driver-foo2zjs printer-driver-foo2zjs-common printer-driver-gutenprint printer-driver-hpcups printer-driver-m2300w printer-driver-min12xxw printer-driver-pnm2ppa
printer-driver-postscript-hp printer-driver-ptouch printer-driver-pxljr printer-driver-sag-gdi printer-driver-splix python3-brlapi python3-cups python3-cupshelpers python3-louis python3-olefile
python3-pexpect python3-pil python3-ptyprocess python3-speechd skanlite skanpage sound-icons speech-dispatcher speech-dispatcher-audio-plugins speech-dispatcher-espeak-ng system-config-printer-common
system-config-printer-udev wvdial xbrlapi
0 to upgrade, 99 to newly install, 0 to remove and 4 not to upgrade.
Need to get 42.9 MB of archives.
After this operation, 147 MB of additional disk space will be used.
Do you want to continue? [Y/n] hell no

[claydoh]

oh noes. although it would have probably fixed it also, it would have been really bad idea and counterproductive, as I just tried to de-bloat kubuntu and it would have put 99% of it back

Yes, but having no idea on what you removed, I suggested the most logical thing, since you completely left out a very important detail -- hacking up the install
This would get you back to a known good state. You have to pay attention to what is being removed along with the desired removals, as different distros have differing amounts of modularity in their packaging. Debian/ubuntu tend to have gobs and gobs of tiny packages in large numbers, compared to others, which makes it easy to miss something important..

Knowing that you had been doing some trimmage, I would have suggested a different meta-package, though you only saved 150mb of drive space. and minimal system resources released.