Every Debian release starts its life as a continuously changing distribution, also known as “Testing”. But at the time you read those lines, Debian Buster is the latest “Stable” version of Debian.

The choice of Debian Buster is well justified based on the fact that any administrator concerned about the quality of their servers will naturally gravitate towards the stable version of Debian. Even if the previous stable release might still be supported for a while, Falcot administrators aren't considering it because its support period will not last long enough and because the latest version brings new interesting features that they care about.