When specifying help in argparse, I often use strings like %(default)s or %(const)s in the help= argument to display default arguments. The syntax is a bit weird, though: I assume it's left over from the days where python strings were formatted with %, but since python 2.6 the standard way to format strings has been using the format() function.
So are these libraries just using the 'old' replacement syntax, or does this come from somewhere else? It's been stated that the % replacement operator will disappear at some point, will these libraries change to the '{}'.format() syntax then?
Yes, the argparse and ConfigParser libraries use the old-style % string formatting syntax internally. These libraries were developed before str.format() and format() were available, or in the case of argparse the library authors aimed at compatibility with earlier Python versions.
If the % formatting ever is removed, then those libraries will indeed have to move to using string formatting using {} placeholders.
However, for various reasons, the % old-style string formatting style is here to stay for the foreseeable future; it has been 'un-deprecated'; str.format() is to be preferred but % is kept around for backwards compatibility.