Comment by ColonelPhantom

I think the point is that if e.g. v7 removes a certain method, but the replacement is already available in v6, you can simply keep using v6 while updating your code to use the new method. By doing all this work on top of v6, upgrading to v7 becomes as simple as 'flipping the switch', and reverting it if issues do arise also becomes trivial.

But yes, I can definitely imagine cases where trying to apply this method leads to a chicken-and-egg problem.