Apple seems to have downloaded the latest macOS Mojave Security Update (2020-005), which was released on September 24th along with the macOS Catalina 10.15.7 update. Apple has also removed Safari 14 for macOS Mojave from download.
Both the Mojave security update and the Safari 14 update caused many issues for those still running macOS Mojave, as described by Mr. Macintosh.
macOS Mojave users who installed the updates have noticed memory issues, slow startup times and Finder stalling, many system.log contents, and crashes when trying to use Migration Assistant, among other issues. Complaints about the update started shortly after it was released, but Apple did not withdraw the updates until yesterday.
Those who had already downloaded Mojave Security Update or installed Safari 14 should soon have a solution in the form of updated software that addresses the errors that were introduced. Going back to a Time Machine backup, reinstalling macOS Mojave or installing acmacOS Catalina also seems to fix the issues.