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The "transitive" compatibility of the earlier versions of the CC-BY has already been well explained by apsillers:

The "transitive" compatibility of the earlier versions of the CC-BY has already been well explained by apsillers:

Why he made the edit when he did? Perhaps, because that's when he checked the compatibility. He actively participates in the CC-BY-SA - GPLv3 compatibility effort - that could very well be the reason that drew his attention to that matter.

The "transitive" compatibility of the earlier versions of the CC-BY has already been well explained by apsillers in c556709:
Earlier licenses are incompatible on their own but are compatible if the work's license allows relicensing to a compatible version. As you can see, this is a general principle not specific to a license.

Why did he make the edit when he did? Perhaps because that's when he checked the compatibility. He actively participates in the CC-BY-SA - GPLv3 compatibility effort - that could very well be the reason that drew his attention to that matter.

The "transitive" compatibility of the earlier versions of the CC-BY has already been well explained by apsillers:

Earlier licenses are incompatible on their own but are compatible if the work's license allows relicensing to a compatible version. As you can see, this is a general principle not specific to a license.

And here's the final update, right from the horse's mouth. The "reference removal" CC clause is an obstacle to declaring CC-anything-GPL compatibility. Whatever the final result would be, as of now, they're deemed incompatible.

And here's the final update, right from the horse's mouth. The "reference removal" CC clause is an obstacle. But as of now, Joshua doesn't consider it a strong enough one to condemn CC-BY as incompatible. (diff to the current licenses.html as of this writing)