Newer rdiff-versions (I am using 2.2.6) started to give deprecation warnings about using a deprecated API, for example when calling:
mscs list-backups <world>
the output will start with:
WARNING: this command line interface is deprecated and will disappear, start using the new one as described with '--new --help'. WARNING: Server will be called with deprecated command line interface to guarantee compatibility. It might lead to a deprecation warning from newer rdiff-backup versions. Use '--api-version 201' (or higher) to avoid it.
As far as I know, this cannot be easily hidden or ignored.