Is this why Apple also quietly released updates for older devices as iOS 9.3.6 and 10.3.4? IIRC Apple has only patched EOL'd iOS releases once before - in 6.1.6 for the ssl gotofail?
Apple's runs out in November 2019 instead of April.
Apple's changelogs are often incomplete at time of release and updated with additional CVEs later. A gps fix sounds like a convenient cover story for an emergency 0day patch for imessage, for example.