Why is CMake's minimum required so low?

cmake_minimum_required(VERSION 3.1...3.20 FATAL_ERROR)

I’m looking at the project to start contributing. Why is the min so low?

It’s at 3.1 only because we haven’t had a reason to raise it. Note that CMake’s own CMakeLists.txt files need to be processed not only by existing CMake versions, but also by the reduced subset of CMake built by the bootstrap script.