The "whole world" in the GP comment is nowhere near accurate. That being said, for starters, Australia still has restrictions on residents leaving without requesting special permission and at least at one point had a ban on re-entry of certain citizens, and the UK previously had a ban on non-essential travel abroad.

Currently in place in Australia: https://www.smartraveller.gov.au/COVID-19

> There's a ban on overseas travel from Australia. You can’t leave Australia unless you get an exemption from the Department of Home Affairs.

An earlier ban in the UK: https://www.bbc.com/news/uk-56493002

> A £5,000 fine for anyone in England trying to travel abroad without good reason is due to come into force next week as part of new coronavirus laws.

Australians have been banned from leaving for over a year, and there are entry restrictions (low rate of people allowed to enter) that make it infeasible for citizens to return unless you pay a lot of money