Gentoo, like Debian, has always been a good distribution for derivatives. It is quite flexible and you can definitely distribute binaries in the case of a commercial derivative. Gentoo has also been used more widely in the embedded world (because it has the ability to lower its footprint). Known derivatives include Chromium OS and CoreOS/Flatcar Linux. Both of them do not use the source model.

Fedora is also very friendly with derivatives and features the ability to upgrade painlessly through atomic upgrades/rollbacks, thanks to rpm-ostree. Something that is shared with Nix. RedHat is mostly a sponsor and Fedora is community-maintained. Unlike CentOS, RedHat didn't get any bad press on how it tries to influence Fedora. Fedora is pretty consistent and quite polished on the desktop side. You also get a distribution who has access to the most influential upstream distributors (including for the graphical stack for example).

Debian/Arch do not offer such things. Arch does not even offer a stable base to start from. Taking a random snapshot of a rolling distribution needs a lot of work. Ubuntu does that with Debian. All Arch derivatives are desktop distributions. It would be interesting to know why Steam did not continue with Debian. Is it bad experience on the previous SteamOS or inability for Debian to move forward?