[ablanton]

PD is great, what other language is working towards a 30 year backward compatibility!

[WillAdams]

(La)TeX

https://www.tug.org

It won't change so that Knuth can finish:

https://www-cs-faculty.stanford.edu/~knuth/taocp.html

[brudgers]

Common Lisp is almost to thirty years because it has a consensus standard.

[monetus]

Really, that is something that sets it apart in my mind. An explicit, rolling two decade+ compatibility effort.

[blacksqr]

Tcl.