The reason for disallowing some undefined escapes is the same as in pattern strings: this would allow as to introduce new special escape sequences. For example:

* \N{...} for named character escape.
* Perl and extended PCRE use \L and \U for making lower and upper casing of the replacement. \U is already used for other purpose, but you have an idea.

Of course the need in new special escape sequences in template string is much less then in pattern string.