[haolez]

On a slightly related note, old Emacs modes (elisp) run incredibly well to this day. There might be working code there that spans over 3 decades.

[Zambyte]

Lisps that support macros are pretty much infinitely backwards compatible. Even across dialects. I wrote a set of macros to be able to program in a subset of Scheme and Hylang, and run that code on either :)

[anthk]

One guy form the PDP-10 'tribe' wrote a Maclisp interpreter in Common Lisp with relative ease. 90% of the code would still run as is, and maybe under Elisp too...