This "restricted domain" is known an effective top-level domain (eTLD)", or a suffix. By Mozilla:
"A "public suffix" is one under which Internet users can (or historically could) directly register names. Some examples of public suffixes are .com, .co.uk and pvt.k12.ma.us. The Public Suffix List is a list of all known public suffixes.
Public Suffix List by Mozilla
The Public Suffix List is an initiative of Mozilla, but is maintained as a community resource. It is available for use in any software, but was originally created to meet the needs of browser manufacturers. It allows browsers to, for example:
Avoid privacy-damaging "supercookies" being set for high-level domain name suffixes
Highlight the most important part of a domain name in the user interface
Accurately sort history entries by site
This repository is an attempt to document the many places where hardcoded domain suffix information lives. Databases, registries, software source code, etc. Some of these registries are publically available, while other registries may be part of closed-source programs or live in a config file maintained by a SaaS provider. All of these registries are considered in-scope for this registry.