The hyphen‐minus is a particularly unfortunate case of
   overloading.  Its awkward name in ISO 8859 and later standards
   reflects the many distinguishable purposes to which it had
   already been put by the 1980s, including a hyphen, a minus sign,
   and (alone or in repetition) dashes of varying widths.  For best
   results in roff systems, use the “-” character in input outside
   an escape sequence only to mean a hyphen, as in the phrase “long‐
   term”.  For a minus sign in running text or a Unix file name or
   command‐line option dash, use \- (or \[-] in groff if you find it
   helps the clarity of the source document).  (Another minus sign,
   for use in mathematical expressions, is available as \(mi.)  AT&T
   troff supported em‐dashes as \(em, as does groff.

The practice of using \- in man pages for the option indicator started because \- looks better typeset, and in nroff it was interpreted as an ASCII minus anyway, since that's all the old terminals could display.  But if you look at groff's an-old.tmac maco file you'll find near the bottom the following section:
.\" For UTF-8, map some characters conservatively for the sake
.\" of easy cut and paste.
.
.if '\*[.T]'utf8' \{\
.  rchar \- - ' `
.
.  char \- \N'45'
.  char  - \N'45'
.  char  ' \N'39'
.  char  ` \N'96'
.\}

This section removes the \- character (along with some others) and explicitly redefines it as code point 45, the HYPHEN-MINUS.  They had to do this for groff's -Tutf8 output mode, because so many man pages use \- for the option character and people looking at man pages in UTF-8 were getting the wrong character when they cut and pasted.