For details on the LXDE desktop please see https://wiki.ubuntu.com/LXDE-desktop
Lubuntu & LXDE - Deprecated
LXDE used the GTK2 toolkit which was replaced with GTK3. The LXDE developers started porting parts (pcmanfm which is both file-manager & handles much of the desktop) of LXDE to GTK3 and noted how much heavier GTK3 was (which was against the intention of L in LXDE), so it was re-ported to Qt5 where it performed much better (pcmanfm-qt). A majority of LXDE developers joined with the Razor-Qt team creating a new desktop - LXQt.
Lubuntu now uses the LXQt desktop.
The last supported release of Lubuntu using LXDE was Lubuntu 18.04 LTS which reached EOL on 30-April-2021.