How can they know that they depend on "a quirk in behaviour of an older version" if a newer version hasn't been released? This sounds bogus.
Of course a newer version has been released. Who said it hasn't been? Eg, the discussion of <=2.5. Hasn't 2.6 been released? Or am I hallucinating?
So, what you are saying is that software X specifies that it is compatible with "2.5 or later" as long as 2.6 hasn't been released, but when 2.6 is released it switches to "2.5 but not 2.6"?
The point is that some packages depend on >=2.5, and others depend on <=2.5. I see no reason to deprecate the "<=" notation.
That's not really what we are talking IIUC. We are talking about behaviour of "2.5" vs. "2.5.0".