afaik the IANA is the body who is authoritative on this type of information. RFC1918[0] defines the address blocks reserved for private routing. From there it's up to various routers and their software to decide whether or not to actually flow the packets over their interfaces or not.

[0] https://www.rfc-editor.org/rfc/rfc1918.html

edit: I think I read the parent slightly incorrectly. I conflated TLD assignment with number assignment. Apologies.