I believe NetBSD would be better for these machines. NetBSD supports old systems quite well and have a release cycle that in cases are longer than Debian.
But, CERN may have some proprietary applications on those systems that force them to use a specific setup which is hard to replicate in other distros.
But, CERN may have some proprietary applications on those systems that force them to use a specific setup which is hard to replicate in other distros.