Is it possible that both Ralf and Fons were just surprised about dropping 99-jack.conf (rtprio & memlock) from the JACK packages, and that they were depending on these and not on their entries in /etc/security/limits.conf? 10-gcr.conf seems to have been there since 2012, and probably has nothing to do with it, and neither does the kernel update.
Personally I think the realtime-privileges package makes sense, as realtime privs are somewhat orthogonal to using JACK, and using the audio group breaks Arch's intended behavior of per seat access to the audio devices. There might also be users that want to use realtime scheduling for other purposes than running JACK.
There was a heads up a few weeks ago on this mailing list, and it also mentions a new optional dependency when updating jack/jack2. It normally pays off to look at the messages during update of the system.
To Ralf, I really think you are making a lot of fuss over nothing.. Nothing has essentially changed, apart from that you have to install realtime-privileges instead of relying on the previous rule shipped with JACK1/2. I can't see that such a change merits so many emails to a mailing list..