How to use Iris on an older installation of Minecraft?

The iris installer and fabric installer allow you to select older versions too. The different install methods are detailed here: https://github.com/IrisShaders/Iris/blob/1.18.2/docs/guide.md

GitHub

(WIP) A modern shaders mod for Minecraft intended to be compatible with existing OptiFine shader packs - Iris/guide.md at 1.18.2 · IrisShaders/Iris