In IEEE 1003.1-2008  and earlier revisions, BRE/ERE grammar allows for
any character to be escaped, but "ORD_CHAR preceded by an unescaped <backslash> character [gives undefined results]".
Historically, we've interpreted an escaped ordinary character as the ordinary character
itself. This becomes problematic when some extensions give special meanings to an
otherwise ordinary character (e.g. GNU's \b, \s, \w), meaning we may have two different
valid interpretations of the same sequence.
To make this easier to deal with and given that the standard calls this undefined,
we should throw an error (EESCAPE) if we run into this scenario to ease transition
into a state where some escaped ordinaries are blessed with a special meaning --
it will either error out or have extended behavior, rather than have two entirely
different versions of undefined behavior that leave the consumer of regex(3) guessing
as to what behavior will be used or leaving them with false impressions.
This is on the tail end of D10315, which should hopefully be wrapping up soon.