The sources from 10.10.2 to 10.10.4 are missing. Is this somehow a preparation for the upcoming T(T)IP? Is the source still accessible for security firms?
Nope. Apple is just extremely tardy in general about releasing source; this has been the case for years.
edit: this even applies to LGPL stuff under the iOS umbrella. The latest version of WebCore available is from iOS 6.1.3; iOS is now on 8.4. I made an explicit request some time ago for the source corresponding to 7.x and 8.0, and after a while got 7.x (apparently they nevertheless haven't bothered to publish it on this site), but IIRC I never got 8.x. Need to check.
edit: this even applies to LGPL stuff under the iOS umbrella. The latest version of WebCore available is from iOS 6.1.3; iOS is now on 8.4. I made an explicit request some time ago for the source corresponding to 7.x and 8.0, and after a while got 7.x (apparently they nevertheless haven't bothered to publish it on this site), but IIRC I never got 8.x. Need to check.