Currently, my app verion 1 is available in all countries. A customer in country A has installed my app.
Suppose I remove a country A from the availability list in App Store Connect when I upload new version 2, what will happen with the app of the customer? I expect that the customer will not get an update
The answer to the first question is the app will definitely continue to work if removed from sale. I'm not sure about the answer to the second question, but my guess is that they can still download the version they already had.
As Chuck says, 1. Yes -- 2. Yes. Only thing about 2 is that the app is still compatible with the OS, later.