What's a useful method to do that?

It includes both ac.uk and go.th.

ac.uk: https://github.com/publicsuffix/list/blob/master/public_suff...

go.th: https://github.com/publicsuffix/list/blob/master/public_suff...

The list doesn't only specify which domains users can register, but it is also used by browsers to determine what constitutes "same origin". This is related to security so even registrars which don't allow the general public to register these domains, it is very much in their interest to add an entry to the public suffix list.

For example, the maintainers of .ac.uk should very much care about the owner of "foo.ac.uk" not being able to set a cookie for "bar.ac.uk", which is partly what the PLS is used for!

For use cases like this, I generally pull from: https://publicsuffix.org/list/effective_tld_names.dat

They'd consider these to be the .hk TLD's: .hk com.hk edu.hk gov.hk idv.hk net.hk org.hk 公司.hk 教育.hk 敎育.hk 政府.hk 個人.hk 个人.hk 箇人.hk 網络.hk 网络.hk 组織.hk 網絡.hk 网絡.hk 组织.hk 組織.hk

They also consider dreamhost.com to be a TLD, so it's probably not the right list for some other use cases.

I’m guessing they probably only put in a handful of common country-level TLDs...