or: 4/ Plan for the inevitable and ensure there is a mechanism for adding functionality without removing or breaking anything.
In other words, try to anticipate what people will need in 3 years and make sure you can easily accommodate it. Don't use bitfields (or, if you do, make sure you leave more than a few bits free).
That's my interpretation, seems consistent to me.
Agreed, CONFIG_EXPERIMENTAL doesn't mean much. I'm not sure it can be improved, though, because pretty much any alternative would fall into the same "technical solution to a social problem" trap. We application programmers love new features, we will adopt them as soon as we can, and we will howl if they break. Alas.