Tested in the latest emacs-25 build as of today.
emacs -Q
C-u C-s
The echo area will show "I-search:".
Earlier it used to show "Regexp I-search:" and that made it clear that we are doing regexp search.
So it looks like that isearch-message-prefix was bound to search-default-mode being non-nil.
I verified that "C-u C-s" shows "Regexp I-search:" fine in emacs 24.5. But that version did not have the search-default-mode.
I believe that the clarity of regexp isearch being on/off is missing, especially when C-u C-s DOES do regexp search and still displays "I-search: " in the echo.
If I set search-default-mode back to "#'character-fold-to-regexp", this issue is gone. So displaying of "Regexp I-search:" should be decoupled from the character-fold.el library.