I feel like most Gentoo folks probably moved over to Nix/NixOS.

The security argument for recompiling from source is addressed by the input addressed (sic) package cache. The customization aspect is mostly covered by Nix package overrides and overlays. You can also setup your own package cache.

I haven't. The Nix language makes no sense to me and there is still nothing akin to useflags. I don't want to override a bunch of packages just to make my system not pull in (e.g.) UI libraries.

Sibling comment aside, I could definitely picture it being a fairly narrow slice of folks who are ideologically motivated enough to choose a niche distro over Ubuntu, Debian, Fedora, or even Arch, but pragmatic enough to still prefer gentoo over Nix.