Initially asked by @tdawson on wrong tracker :

The current CentOS Stream koji build system[1] uses mock 2.8.
In that version of mock, when it creates the mockbuild user it uses -n as one of it's options.
useradd -n has always been an alias for useradd -N
In Fedora shadow-utils-4.14.0-1 this alias was removed, and it now only has useradd -N
Because the -n option is no longer available, builds for CentOS Stream 10 no longer build, but fail with

 
ERROR: Command failed: # /usr/sbin/useradd -o -m -u 988 -g 135 -d /builddir -n mockbuild
Please patch mock on CentOS Stream koji so that it uses -N instead of -n