... and with the result of #6 also _ have special meaning in syntax-rules patterns, so they are not treated as pattern variables by default.
However their behavior when used in the literals list of syntax-rules is ambiguous, and simply breaks in most implementations.
Rather than breaking, it makes sense to go ahead and treat them as normal literals, overriding their special meanings.
In particular, there are many existing R5RS macros which make use of _ in the literals and are thus broken outright by #6. Allowing them as literals fixes these macros.