Slackware is the oldest distro still being actively maintained. It’s a fully featured and modern distro. KDE Plasma is the default DE but you can also choose XFCE, fluxbox and a few others out of the box or install whatever you like. It’s a different way of doing things and it’s not for everyone. But I’ve been using it more or less continuously since 1998 or 1999.
It has two versions. LTS is currently version 15. Slackware current is a bleeding edge rolling release. Eventually, they will take current and build a release candidate and then that will be the upgraded LTS version. There are also live boot ISOs available for anyone to try.