Currently the 2 commands spo hubsite register and spo hubsite unregister have a remark stating that the API is in preview and is subject to change. However I think the API is stable right now. When I consult the docs, I don't see any warning saying that the API is in preview.
https://learn.microsoft.com/en-us/sharepoint/dev/features/hub-site/rest-registerhubsite.method
https://learn.microsoft.com/en-us/sharepoint/dev/features/hub-site/rest-unregisterhubsite-method

Another thing about the docs: you should be SharePoint admin or Global admin to register or unregister a site as hub site. Usually when you need special permissions, we also explicitly show them in our docs. In this case the permissions are missing.

Things to do

For both spo hubsite register and spo hubsite unregister:

• Remove the API preview remark from the docs
• Add remark To use this command you must be a Global or SharePoint administrator.
  Example:
  

  cli-microsoft365/docs/docs/cmd/planner/tenant/tenant-settings-list.md

  Lines 17 to 18 in 1f313f7

  	!!! important
  	To use this command you must be a global administrator.