Aside from CL implementation bugs. The implementations had lots of them back then, even the commercial ones. Nowadays, they are much better. Decades of effort to make an implementation bug free and shiny will have its effect.

That really is amazing. Is it true, even "usually", for most types of code? Both forwards and backwards compatibility.. it's like the holy grail.

For most types of code, other than things like code which has dependencies on implementation internals (like extensions in SBCL for instance), or uses FFI to access platform libraries that didn't exist in 1995; that sort of thing.

ANSI CL was last ratified in 1994; there has been no standard language change, and the community language improvements are just macro libraries. You should be able to take your alexandria utilities library, or optima pattern matching or whatever back to 1995.

There are a few community improvements that are language extensions. The standard had no notion of multithreading, MOP was not in the standard, and packages really needed some way to avoid collisions of short package names (which lead to the de facto standardized package local nicknames.) Pathnames may also have seen some de facto standardization; they were poorly specified in the standard.