2.1 is described as the current standard. It contains sections marked VOLATILE which indicate sections which are under active discussion and/or likely to change in some future version of the standard.. I'd read this as meaning 2.1 is the standard as is, but that as the volatile sections might alter in later versions that may be a good reason to avoid the contents of those sections if possible.

The long absence of Chris Walshaw from taking charge of the standard meant that various people (implementers usually) added to and modified the language without any overall strategy. The 2.0 draft was at least an attempt to rectify that, though it seemed to remain a permanent draft. I subscribed to the mailing list for some time and discussions just seemed to go on and on. With no one really in charge to make a final decision on points that everyone would accept it was probably destined never to get beyond draft status. (It's a lesson from the development of computer languages in general that you really need an agreed standard otherwise developers invariable diverge in their implementations, dropping things they feel are too hard, not necessary or they don't agree with, and adding things that in their opinion would be good to have. It leads to things getting messy for users when they get the the point of needing to exchange programs in different environments). In fairness, Chris Walshaw probably never foresaw such widespread use of abc when he invented it.

Mick