I disagree that the documentation is at fault.  This is known to break existing code, e.g. http://bugs.python.org/msg281496

I think it's not correct to change the documentation but leave the error-raising behavior for 3.6 because the deprecation was never documented in 3.5 so this will look like a gratuitous regression.  issue27030 for reference.

I also question whether it makes sense for such escapes to be illegal in the repl argument of re.sub().  I could understand for this limitation in the pattern argument, but that's not what's causing the error.