Since version 1.12.3 is a maintenance update of the 1.12 branch, it includes all the major "final" features added to the game:
The 1.12 series updated the game to Unity 2019.4.18f1 LTS , providing better stability and support for modern hardware. Archivo: Kerbal.Space.Program.v1.12.3.3173.Incl...
Introduced the "Slim" suit, a sleek design inspired by modern private aerospace sector EVA suits. Since version 1
Scattered new discoverable launch sites and easter eggs throughout the Kerbol system. or other critical mission events.
Added limited rotation to docking port collars, allowing for precise vessel alignment after docking.
A built-in system to set reminders for maneuver nodes, apoapsis/periapsis crossings, or other critical mission events.