The whole point of this issue is that it is not installed on Debian, which means also not on any derived distros.
The point of this issue is to: * Allow people compiling Geany to specify the Python binary. * Use the proper default binary on Windows by default.
I don't think it's meant to necessarily change the existing behaviour on non-Windows, just to allow more flexibility, given that some distros (like Mint 20 apparently) are not providing `python` binary/link, so they no longer have to patch Geany's build system, they can just update the package build options to specify `--with-python=python3`.
That said, one could open a new Issue/PR to change from `python` to `python3` in the `filetypes.python` file and `main.py` templates and such.