Apple has updated macOS Catalina, probably for the final time except for security-only fixes. The update – the sixth since Catalina debuted in October 2019 – is also a sure signal that one of its forebears, macOS High Sierra, is now close enough to retirement that it may as well be shopping for loud shirts, ugly sandals and an enormous RV.
Catalina 10.15.6 sports a very short list of improvements to Apple News, fixes two bugs and adds an option to optimise video streaming on HDR-compatible Mac laptops. Apple said that the option would extend battery life for aggressive streamers.
The update also patches 19 security vulnerabilities in 16 of Catalina's components, ranging from Audio and Kernel to Mail and Wi-Fi. Most of the flaws, absent a fix, might have let criminals run attack code on a compromised Mac.
Going forward, macOS 10.15.6 will likely be the final non-security update to Catalina, the still-current Mac operating system. For the last three cycles – Mojave, High Sierra and Sierra – Apple released six non-security updates to each edition; the four before that topped out at five updates apiece.
Likewise, the Security Update 2020-004, also issued Wednesday, was almost certainly the last for macOS 10.13 High Sierra, the operating system Apple launched in September 2017. Apple supports the latest edition of macOS (call it "n") and the two before that ("n-1" and "n-2). When a new edition launches – this year's upcoming Big Sur – the oldest (n-2) falls off the support list.
That n-2 is currently High Sierra.
More telling, Apple has a habit of releasing the last non-security update for the then-current edition about 10 weeks before the successor goes public. Last year, for instance, Mohave's final non-security update, 10.14.6, made the scene on July 22, or 77 days before Catalina's release.
In 2018, High Sierra's last non-security update, 10.13.6, appeared on July 9, 77 days before Mohave's launch. And the year before that, Sierra's final non-security update, 10.12.6, shipped July 19, or 69 days before High Sierra debuted in final form.
The three-year average gap between one edition's final non-security update and its successor's release: 74.3 days.
The past three macOS upgrades were all released on a Monday: Catalina on Oct. 7, 2019; Mojave on Sept. 24, 2018; and High Sierra on Sept. 25, 2017. If Apple sticks to that pattern, it will post Big Sur on the Mac App Store on Monday, Sept. 27, 75 days after the appearance of Catalina's 10.15.6.