I am wondering if we should remove the 'Optional' part and add something like 'upstream'. Some people use Hiera to change settings in modules, and the current way make it impossible to revert to upstream on one host if another level in the hierarchy set it to something else.

Yes, I have that in my mind, but it'll be breaking change (potentially, if someone do class { 'caddy': install_method => undef }). So far I'm trying to keep backward compatibility.

I mean, I'll change this, just in different PR.