Coreutils gets updates regularly! https://gitweb.git.savannah.gnu.org/gitweb/?p=coreutils.git
Even `ls` gets news flags from time to time.
I think "stopping" is great for software that people want to be stable (like `ls`) but lots of software (web frameworks, SaaS) people start using specifically because they want a stream of updates and they want their software to get better over time.
The bar for adding new options, especially short options, is quite high for coreutils. We have a (likely outdated) page of rejected requests [1]. Some of the changes people have strong feelings about...
[1] https://www.gnu.org/software/coreutils/rejected_requests.htm...