All I’ll say about this is that for the foreseeable future, SXEmacs will remain 100% backwardly compatible with XEmacs where emacs lisp is concerned. An emacs lisp package or library that runs on XEmacs will run on SXEmacs.
If you find a place where this isn’t true, you should report it as a bug, See Bug Reports.
If you find areas where SXEmacs is incompatible with GNU/Emacs at the emacs lisp level, that is an issue between GNU/Emacs and XEmacs. That doesn’t mean that we won’t ever port GNU/Emacs things to SXEmacs, we’ll just do it in a way that doesn’t break compatibility with XEmacs.