Rather than cluttering up the newer version what do you think of having slime include the older version and have the swank loader conditionally load old or new depending on the ABCL version?
There is also slime maintenance cost associated with having it be backward compatible.
An alternative is to check in a compatible version of slime as part of each ABCL release. This requires a bit of thought about managing the case where someone wants to run multiple different lisp implementations in the same emacs, thought I'm.not sure how prevalent this is.
Alan