As far as I understand it, the earliest communication was 2017/11/24, which they addressed as it appeared that GnuPG wasn't vulnerable as it would fail with an error message when processing the exploit as it was known in November.
A few days later, they got an email asking for a phone call, which wasn't followed up upon--I'm not sure to what extent this is GnuPG's fault, whether this is a usual way of communicating a serious vulnerability. It would've been better if they called em back, definitely goes for both parties.
Then after a few months of silence, 2.5 weeks ago, they received another email with a different title that, according to Werner Koch, was too redacted to act upon:
On 2017-11-29 we got a short mail asking for a phone call. It might be
that I did not reply to that but in any case my office phone number is
easy to lookup. I did not get a phone call.
Since then we have not seen any more communication - not even about the
proposed coordinated public disclosure. Thus I closed this issue in
December and forgot about it.
On 2018-04-27 I received another paper via a Kmail developer which had a
different title than the one from November
*** DO NOT PUBLISH OR SHARE ON PUBLIC MAILING LISTS ***
Efail: Breaking S/MIME and OpenPGP Email Encryption using
Exfiltration Channels
and no author names etc. The GnuPG team discussed this but did not see
that any action was required. In particular because due to the
redaction we were not able to contact and help the developers of other
MUAs which might be affected.
I mean yeah,
in theory, BOTH parties could have been better at communicating and following up on it. But both parties are probably very busy and this kind of "no you call me back" is the sort of thing that falls through if you have to make choices about what to spend limited time on. That's why there exists
protocol and
coordination to tell something is really serious/urgent.
It does seem to me that the security researchers were a bit too trigger-happy on the redacting of information, in particular unnecessarily so in private emails to the security folk they expected to act on the vulnerability.