Hello,

I was excited to see mention of dir-locals-file-2 in NEWS, but then
recalled that that feature was added and then removed because of
performance issues.

http://git.savannah.gnu.org/cgit/emacs.git/log/?h=emacs-26&qt=grep&q=dir-locals-file-2

References to dir-locals-file-2 still need to be removed from:

km²~/downloads/:git/emacs> rg 'dir-locals-file-2'

ChangeLog.2
9419:   (dir-locals-file-2, dir-locals--all-files): Remove.
9935:   (dir-locals-file-2): New const.

etc/NEWS
360:See the variable 'dir-locals-file-2' for more information.

etc/DOC
46108:See also `dir-locals-file-2', whose values override this one's.
46109:See Info node `(elisp)Directory Local Variables' for
details.Vdir-locals-file-2

lisp/files.el
3868:See also `dir-locals-file-2', whose values override this one's.
3871:(defconst dir-locals-file-2 ".dir-locals-2.el"
-- 

Kaushal Modi