Paul Eggert <eggert@cs.ucla.edu> schrieb am Sa., 30. Dez. 2017 um 07:19 Uhr:

Why not simply take the new interpretation for backquotes? That would be simpler
than adding a backward-compatibility variable that I suspect nobody will ever need.

As discussed in http://lists.gnu.org/archive/html/emacs-devel/2017-10/msg00051.html, we (mostly) agreed to first turn them into errors because otherwise the interpretation would suddenly silently change.