TL;DR: 4.3.1.x is a Premium-only development (beta) branch.
The year is 2020 and large parts of the WordPress ecosystem still do not understand Semantic Versioning. There are workarounds for most of the issues this causes, but the end result isn't a complete solution and it's an unnecessary hassle.
The solution is to borrow an idea from early in the history of Linux:
- Odd numbers: Development
- Even numbers: Release
Thus, 4.3.1 is the Development branch for the future 4.3.2 Release.