Apple has just released a set of new updates for macOS Mojave 10.14.6 to address earlier problems with the Safari 14 update and Security Update 2020-005. These should ensure that all Macs running 10.14.6 have Safari 14 installed, together with the fixes of the later Security Update, without suffering problems.
Mojave 10.14.6 Supplemental Update contains Safari 14, resolves an issue which could impair performance, fixes stability in Migration Assistant, and problems creating new user accounts. A new version of Mojave Security Update 2020-005 then addresses the vulnerabilities which the previous update had tried to address, but this time without any adverse side effects.
Currently, Apple hasn’t released either as a standalone update. Users are best checking which updates Software Update recommends for their Mac(s), as that should determine which of the two installers to download and run, which differs according to which of the superceded updates had already been installed. If you hadn’t installed either Safari 14 or Security Update 2020-005, then you should receive both the Supplemental Update and the revised Security Update 2020-005. If you had installed both the earlier updates, then you should receive only the Supplemental Update. Other combinations are offered if you had only installed one of the previous updates.
When standalone versions of these updates become available, I will look in more detail at their contents.
Further information is available from Mr Macintosh, who also advises that these revised updates do appear to address the problems which had resulted from the original releases.
Apple hasn’t yet released any further notes on these updates, and its security notes merely report that the Supplemental Update contains Safari 14 and its fixes.
Thanks to Hawk for drawing my attention to this.