At the time of writing (October 2022) all countries are now open in respect of COVID19, except for China. It looks like we are moving to a post-COVID19 world.
Some countries require you to have a COVID19 vaccination – see the relevant country’s website. If you are unvaccinated and fall outside of the various categories (for instance, having a medical exemption), you will be refused boarding.
As always, check with the countries’ government.