> I came across this piece of code in Lib/string.py:146:

That may be artifact from the days of ASCII strings and may or may not have analog with the bytes data type.

> But I'm not exactly sure string.Template should be in 
> Python 3 at all.

It still has valid use cases (i.e. exposing safe templates to the end user).  And it has features that aren't present in new-style formatting.

Please learn an aversion to deprecating things (it almost never a good first reaction to seeing code in the standard library).  It almost always breaks someone's code and makes migrating to Python 3 more difficult.