It has always been recommended to have port 80 open world-wide for the HTTP Challenge. In the past you could get away with blocking non-USA countries but this was only by luck. Let's Encrypt has long validated from both US and non-US countries.
The first two other threads linked to in the first reply to you explains the recent change and the technical details behind it.
While I think you should read those in total I think you will find this topic in the FAQ wiki a good start based on your other comment