From what I understand, there are more now that support that level of customizability, such as Arch.
What I really love about Gentoo, and why I still use it, is that it just happily upgrades without forcing things on me almost all the time. Sometimes I have to eselect news read and make a decision about something that's going out of support, but that's very uncommon. It's a rolling release that doesn't suddenly tell me I'm using systemd or insist that now I have to use nginx or whatever.
And since it's from source, all the compiler toolchain bits are already installed.
And I can funroll my loops and fomit my pointers.
Everyone should install Gentoo at least once. It's not hard, the documentation is excellent (up there with Arch) and you learn things. You may not want to, but you do.
Oh, and that color documentation? That made such a huge difference! Everything else was HOWTOs and black and white and here was well-written documentation with colors to help you understand the fixed and movable parts of the commands; chef's kiss.
Ooooo sys-kernel/gentoo-sources-6.1.28 just dropped, time to go! I'm coooooooompiling!
Also genkernel is one of the nicest interfaces to rolling your own kernel you're ever going to encounter. It's a joy to work with.