If you specifically require the 18.04 base, you may have to look into the following: KDE Neon Bionic 18.04 with Plasma 5.13.5 Released

Official downloads for the 18.04-based ISOs are on the main KDE neon download page . The project typically moves to the next Ubuntu LTS base (such as 20.04 or 22.04) and stops maintaining or hosting older images once the rebase is complete.

KDE neon 18.04, famously based on Ubuntu 18.04 "Bionic Beaver," represented a significant milestone for the project when it was released on September 26, 2018. It combined a rock-solid Long Term Support (LTS) foundation with the latest, bleeding-edge Plasma desktop and applications. Can You Still Download KDE neon 18.04?