> In fact, no citizen of any country can be denied entry to their country of citizenship

This seems like splitting hairs—it's not called denying entry, it's called revoking citizenship.

Splitting hairs on the hair splitting - a country cannot revoke someone's citizenship if doing so leaves them stateless. At least, this is the UN convention of human rights apparently. Of course, in reality, the country doesn't deny entrance, but could just as easily detain upon entree.

You're right but sometimes they try to argue that they belong to some other state like the UK did in "Begum v Secretary of State for the Home Department [2021]"

There are plenty of countries who have not signed on to the UN Convention on the Reduction of Statelessness, including the US. So, yes, you can be left stateless by the US revoking your citizenship. However, pretty much only treason or voluntary renouncement could potentially lead to such a situation, and treason hasn't been charged in the US since the immediate aftermath of WWII.

The U.S. border does and will refuse entry if you do not have proper identification.

Not if you're a US citizen.

"If you’re driving or walking back into the US from Canada or Mexico, and you don’t have your passport with you, be prepared for some hassles. Although you won’t be denied entry to the United States, you will be taken out of line to be interviewed by US Customs and Border Patrol officers. Your return to the US will be delayed until the officials are satisfied that you are a US citizen."

https://passportinfo.com/blog/new-passport-fines-on-us-borde...

You can clearly be denied boarding a flight by an airline if you don't have the proper documentation.

I had confirming experience on Cruise Ship operating in the Gulf of Mexico. Didn't have passport with me as I went through immigrations. I looked at the selection of immigration officials and was of course directed to the most intimidating official. When I told I didn't have passport he threatened to have me beaten. Happily he was joking. I went through with no issues at all. However, I also suspect skin color matters in these outcomes.

> You can clearly be denied boarding a flight by an airline if you don't have the proper documentation.

Yes -- airlines are already fined significantly if they knowingly admit someone undocumented, with wrong documentation, etc by the destination immigration authorities, so it's in their best financial interest to have their own internal strict controls as to who can get on and who can't.

> In fact, no citizen of any country can be denied entry to their country of citizenship

Not Australia. During the height of the pandemic, there was a period of time that the government made it illegal for Australian citizens to come here from India.

Would try Australian government deny them entry, or let them enter and immediately arrest and isolate them?

I actually flew into Australia during the Pandemic.

I think there was a technicality there - the Australian government just cancelled all commercial flights from India. If you wanted to sail your own yacht, I'm sure you could have come int.