.mk appears to allow for it: https://marnet.mk/wp-content/uploads/2023/01/pravilnik-mk-mk...

“The name of the .mk domain consists of a minimum of 1 (one) and a maximum of 63 characters.”

ICANN might have set some rule for .com/.net/.org but it's not universal for all tlds

They’re not reserved, pretty easy to obtain if you have money, starting from less than $1k.