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...
Version was the last major content update for the game. The specific build v1.12.3 focused on refining the significant additions of the 1.12 series while ensuring stability as the developer, Squad, shifted focus toward the sequel. Key Features of the 1.12 "Final Approach" Series Since version 1
Added customizable firework launchers (Starshot-8 and Starshot-32) for mission celebrations. Technical Context Archivo: Kerbal.Space.Program.v1.12.3.3173.Incl...