It is unfortunate that the deprecation note did not explicitly mention replacement templates as well as regexp patterns.  While there are good arguments to be made for either case, I think it makes more sense to treat the replacement template as following the rules for strings rather than for regexps which would argue for making the proposed change to cause only a deprecation warning in templates for 3.6 - although I'm not happy about making such a change at the last minute.  Serhiy, please push to 3.6 for rc1.  There should also be a note in the What's New for 3.6 document.