18.04 is the last release that comes with LXDE, wikipedia even lists the requirements System requirements for Lubuntu 18.04 LTS included a minimum of 1 GB of R

18.04 is the last release that comes with LXDE, wikipedia even lists the requirements System requirements for Lubuntu 18.04 LTS included a minimum of 1 GB of RAM, although 2 GB was recommended for better performance, plus a Pentium 4, Pentium M, or AMD K8 CPU or newer.[139] The RAM requirements increased from Lubuntu 17.10.