As expected, Apple has just released macOS Catalina 10.15.6 Supplemental Update, which it claims fixes the memory leak which could occur with virtualisation, and resolves an issue in which iMac Retina 5K 27-inch 2020 displays could appear washed out when waking from sleep. The update is over 3 GB.
The standalone installer is now available from here. Apple might incorporate it into an updated 10.15.6 Combo installer in due course, although often Supplemental Updates are left on their own. The 10.15.6 Combo update, originally released on 15 July, is available here.
This update brings firmware updates only for Macs with T2 chips: I have updated those versions in the SilentKnight database, and in my article here.
Postscript: This does appear to fix the kernel zone memory leak which was causing panics when running VMWare and VirtualBox VMs, and seems to have restored stability to memory usage here too. I’ll provide further details when this iMac Pro has been running longer following installation of the update. But if you’re wondering whether to install this or not, don’t delay – it appears to make 10.15.6 much more stable.
Updated 0925 13 August 2020 confirming firmware updates.