They have been deprecated for a decade now.
* src/lread.c (Fload): Don't use record_unwind_protect to warn about
old-style backquotes any more. They now generate a hard error.
(read1): Signal an error when detecting old-style backquotes. Remove
unused label.
(syms_of_lread): Remove unused internal variable
'lread--old-style-backquotes'.
(load_error_old_style_backquotes): Rename from
'load_warn_oldstyle_backquotes'. Signal an error.
* lisp/emacs-lisp/bytecomp.el (byte-compile-from-buffer): Remove check
from byte compiler. It isn't triggered any more.
* test/src/lread-tests.el (lread-tests--old-style-backquotes): Adapt
unit test.
* test/lisp/emacs-lisp/bytecomp-tests.el
(bytecomp-tests--old-style-backquotes)
(bytecomp-tests-function-put): Adapt unit tests.
* etc/NEWS: Document change.