Gambit Scheme claims to be the third-oldest, active Scheme implementation. It seemed like an interesting case study for the Lindy Effect, so I tracked down a few Scheme implementations for comparison.
This list is sorted from oldest to youngest. A ☠ indicates implementations that had fewer than 5 commits to their repository in 2021.
Scheme was invented and R0RS published in 1975. R1RS is dated 1978. Three implementations have survived from Schemeʼs first decade: MIT, Chez, and Gambit.
Iʼd love to add notable, old, dead Schemes to this list. Mail me if you know of any: