>From what I understand, there are more now that support that level of customizability, such as Arch.

Kind of (having previously used gentoo but now using arch). Gentoo has the philosophy of 'here, you can build this package exactly how you like, and then configure it'. Arch is more like 'here's the package built as close to how the developers intended, and here's how to configure it'. You can build stuff yourself on arch but it doesn't encourage it, and a lot of decisions are made explicitly because they makes maintainers lives easier, not provides features to users.