> If this is generally unsupported, can we then remove it from the doc and
Yes. Do you happen to know where it's documented?
"You can use the same syntax for punctuation characters, but it is often a good idea to add a ‘\’ so that the Emacs commands for editing Lisp code don’t get confused. For example, ‘?\(’ is the way to write the open-paren character. If the character is ‘\’, you must use a second ‘\’ to quote it: ‘?\\’."
Here I would put ( and ) into the same category as \, so that it's not only "a good idea" to escape it.
"However, you should add a backslash before any of the characters ‘()\|;'`"#.,’ to avoid confusing the Emacs commands for editing Lisp code."
Here I would make escaping mandatory for the first three characters (or even for all of them).
> have the reader print a warning (or reject it altogether)?
We can't reject it, because I'm pretty sure there's code out there which
uses it. I'd accept a patch which adds a warning for it,
OK, I'll try to come up with a patch.