Most popular GUI stuff is from universe, as are quite a few dev tools. Some examples: Gimp, Inkscape, pip (and a ton of python packages), most of gnome, a big chunk of KDE, htop, mariadb, etc.
See for yourself grep -h "^Package:" /var/lib/apt/lists/_universe__Packages | awk '{print $2}' | sort -u
Or to see only what you have installed from Universe: comm -12 <(dpkg-query -f '${Package}\n' -W | sort) <(grep -h "^Package:" /var/lib/apt/lists/_universe__Packages | awk '{print $2}' | sort -u)
A big repo isn't always better.
All LTS distros fix only some core packages sporadically as no one is able to back port all the patches esp. since most packages do not use CVEs and just fix bugs on the go. "Stable" for non-rolling distributions simply means "horribly broken and outdated".
I don't know where this sense of "stable" in the community comes from. Software isn't perfect and gets fixed all the time. Yes, there are packages with different maintained stable branches that you can pin for your LTS distribution but this is by far the minority. For the other stuff you constantly have to work around missing features or existing bugs. E.g., why do I have to compile "jq" by myself just because the outdated package crashes on certain inputs?!