As long as users explicitly tag their ABCs with the version they're coding to, the "volatile" features should be safe to use. In fact, if they're coding to any version prior to 2.1, things should work properly without a version tag, since the standard says that converters should default to 2.0 behavior, for backward compatibility. (Whether converters will actually follow suit remains to be seen.)

Similarly, since abcm2ps is the most widely used program for formatting, I would hope that developers would include support for a core subset of its extended features for any ABC versioned explicity or by default as 2.0 or before. The paucity of the "official" standard in regard to format control should not prevent users from using badly-needed features that already exist in this alternate de facto standard.