Today Apple released the 5th Supplemental Update for macOS Catalina. This time around, the fix is for memory leaks/KP's with virtualization software. The other fix has to do with the brand new 2020 5k 27-inch iMac, graphics washed out after waking from sleep.
Apple also unified the full installer (adding 2020 iMac) and updated the 10.15.6 combo and delta updaters.
Apple released the supplemental update with a build version of 19G2021. A four digit daily build number is usually only reserved for a hardware specific or forked build. So it's pretty strange that they would do that.
https://mrmacintosh.com/catalina-10-15-6-supplemental-update-19g2021-released/