Messages and patches can be unlinked from an issue (and that action recorded in the history list) but not edited.  Proofread before clicking.

Originally, only a few module docs had a source code link where the source code was thought to be especially useful.  When these links were determined to be useful, more have gradually been added as some developer thought of it or as some user requested, as you did in #22528.  I don't know if any requests have been denied.  I also don't know if there is a written policy anywhere.  To me, it might be easier to link all python sources and be done with the issue.