As I wrote on XEmacs-Beta:
For example, if the APIs that use a customization change, but its
default value doesn't, then :version doesn't change but its meaning
does. I'm not even sure if they update :version when additional
non-default values are added to a customizable variable. Also, if a
variable is deprecated or deleted, that won't change :version AFAICT.
Finally, the most interesting changes are in functions, but those
don't have a :version at all!
Might be interesting to implement that, if it can be done
automatically. I wonder if hg has a git diff equivalent (ie, that
reports the function containing changes) that could be warped into use
here, eg as part of the DOC file.