Joshua Gay’s edit looks sloppy at best: I found at least 2 clauses and a section in http://creativecommons.org/licenses/by/4.0/legalcode that look incompatible with any GPL.
I’ve written a letter to the FSF asking them to clarify this matter and the properties of the edit.

This is what I just got in reply, from none other than Joshua himself (nested blocks are his quotes from my letter):

Hello,
Thank you for writing.

• It doesn’t mark the license as “compatible with the GNU GPL or FDL”
  with the left-side line.

The color doesn’t specify a version number of the GPL. I have been
waiting for confirmation from our general counsel that CC BY is
incompatible with GPLv2 and the FDL before updating the color on the
left hand side. Hopefully that will be soon.

• It doesn’t comment on this decision as if it’s something obvious.
  Which it is not:
  • The http://creativecommons.org/licenses/by/4.0/legalcode#s3a3
    restriction looks drastically incompatible

First note that both licenses make use of words like reasonable and
within reason. Interpretation should be done with that in mind. Here is
our position on this as I understand it. You are right that we should
have a more formal statement and I will try to get one prepared that
we publish on the site.

GPLv3 7(b) states that you can add terms “Requiring preservation of
specified reasonable legal notices or author attributions in that
material or in the Appropriate Legal Notices displayed by works
containing it;”

The FSF interprets this as to include a licensor doing something such
as removing certain parts of the reasonable legal notices or marking
them in ways as different from the original.

• http://creativecommons.org/licenses/by/4.0/legalcode#s4 grants a
  set of rights whose relation to those granted by the GPLv3 is very unclear

How are they unclear? I do not see what is unclear.

• http://creativecommons.org/licenses/by/4.0/legalcode#s2a5B
  appears to forbid relicensing which is a requirement for GPLv3 compatibility as per
  http://www.gnu.org/licenses/gpl-faq.html#WhatDoesCompatMean

A person does not need to relicense a work to create a combined work
with the GPL. A CC BY licensed work that is incorporated into a GPL
licensed work would retain a copy of the CC BY license and all CC BY
notices. One would simply add to the work as a whole and in relevant
plces notices about the GPL and a copy of the GPL.

CC BY does not restrict adding additional terms so long as those
additional terms do not restrict “exercise of the Licensed Rights by
any recipient of the Licensed Material.”

I will see about adding these clarifications somewhere.
Thanks again for emailing us.

Joshua Gay
Licensing & Compliance Manager
Free Software Foundation

This is the reply I got on further clarification of CC#s3a3. He believes it’s compatible by doing a narrow interpretation:

The incompatibility as it appears to me is that GPL gives no right to
change any terms
for a licensee who has already received a license (or receives it from
an older copy) –
since a license is irrevocable and is granted on the terms that
accompany a specific copy.

So, your interpretation of this CC clause is that it only applies
to the moment one receives a copy from the copyright holder, not to
any later moment
or receiving it from any other party?

That is my current understanding. Do I have a reason to think it would
mean at a later date? I assumed such a request would need to be made
alongside the license itself. I will ask Creative Commons what their
interpretation is.

I am not a lawyer and I have no ideas how courts would interpret this.
But, I’m not sure how one could reasonable expect a work to be put
into the wild, modified and remixed for a period of 75 or 80 years and
then it be reasonable that a licensor could come along and ask for all
of those notices to be removed.

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)

I suspect the clause does have such a use in mind. Its intended use
appears to be the case where a copyright holder sees their work used
in
a context that they deem inappropriate – so they wish any reference to
them
removed so that their perceived reputation doesn’t suffer.

Creative Commons sent me a clarification and it is the case that the
intent of the license is so that a user could ask after the fact.
However, they explained that “to the extent reasonably practicable” gives a person a lot of flexibility in being able to simply deny the
request of the original licensor. I am interested in finding out some
other legal opinions on how strong the language “to the extent
reasonably practicable”. Like how easy it is to make that argument as
a licensee. I believe this is will be the next question in the public
discussion mailing list. I will also open up an internal discussion at
the FSF and with our lawyers to make sure they think the language
“reasonably practicable” is sufficiently strong.

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.