I’d wager most modern rocket engines are 3D printed. At the very least SpaceX uses 3D printed engines. It greatly enhances the ability to cool the engine using the supercooled propellant and the technology to achieve it isn’t exactly cutting edge. 3D printing metal using SLS has been around since the eighties
Laser powder bed fusion (L-PBF) of polymers was only in its infancy in the late 80s. This was research out of UT Austin that would eventually lead to DTM. Proper L-PBF of metals (fully melted, fully dense parts) didn't come about until the late 90s in the research world. L-PBF of metals wasn't really commercialized until the 2000s.
In any case, this is lightning speed for an industry that still produces something initially designed in the 60s (Boeing 737).
