There was a long discussion 14 years ago when mb-depth.el was
implemented about making it a defcustom or not:

[1] https://lists.gnu.org/archive/html/emacs-devel/2008-08/msg00025.html
[2] https://lists.gnu.org/archive/html/emacs-devel/2007-06/msg00304.html

These are the only references I could find about this topic. Perhaps
today the rationale for this decision is different and this patch can be
applied. If the decision is to keep it as a defvar, please close this
bug report.

Regards,
Gabriel