Thanks for pointing it out. I just searched for the current source tree
of master branch for ocurrences of "(default %s)", so I am not sure if I
changed some package that shouldn't be update. If so, I will promptly
update the patch.
Perhaps project.el could use a similar fallback that exists on
cperl-mode.el (commit 3a95f4fc12df0a570ad413e099a22bf901b7346d):
;; Available in Emacs 28: format-prompt
(defalias 'cperl--format-prompt
(if (fboundp 'format-prompt) 'format-prompt
(lambda (msg default)
(if default (format "%s (default %s): " msg default)
(concat msg ": ")))))