you can use master or any other branch, but without warranty.
The usual approach (and the one favoured by GitHub) is: features are developed in branches. When they are considered stable enough they are integrated into the master (pull request). Hence the master is kind of "latest greatest", but with a certain risk of being spoiled. There are also forks from other user with their own masters and branches. Don't confuse them with the ones from the KLayout project.
But eventually, when the master is considered to have a sufficient quality, a release is made by creating a release tag (https://github.com/KLayout/klayout/releases). A release is a snapshot which is then turned into Windows binaries, Linux packages, PyPI wheels etc. This this is a lengthy process and I don't want to do this too often.