Ref T13216. See PHI985. This config option once controlled adding a Herald transcript link to email. However, this was never implemented in a generic way and was removed from revisions in D8459 and from commits in D10705. No one has noticed or asked for this option for several years, so this is probably a good opportunity to simplify the software and reduce the total amount of configuration.
If we did want to pursue this in the future, I'd generally prefer to make it part of the mail detail page (/mail/detail/12345/) anyway.