To reply to one of my own questions:
I have just experienced upgrading a PC from a version of Avast that used the Feature called “NG” (apparently the prior implementation of Secure Virtual Machines) to the newest version. The old Avast had set the System Protection on the C: drive of the PC to 50% of available hard disk space (on a 1 terabyte drive) and it had rapidly accumulated a disk footprint that broke the regular image backups to a shared network drive. I changed the Avast install to disable the “NG” feature (along with Grimefighter, Avast Gadget, Avast Remote Assistance, and Home Network Security), then reset the System Protection on C: back to a more reasonable 2%. After upgrading to the latest version of Avast, everything I had turned off remained off, but the new Secure Virtual Machines feature was turned on and the System Protection on C: was cranked back up (now to “only” 20% - 200GB). So now I know that any client who does as I’ve suggested and upgrades their Avast when requested may break their backups and need my assistance.

I, like many people in these forums, have always appreciated Avast for its virus prevention and remediation abilities and its relatively light footprint. But it’s challenging to continue recommending it as its footprint grows and its behavior causes unintended but damaging consequences. Let’s hope this is just a slight bump in the road and Avast doesn’t go the way of so many other A/V vendors where the reach clearly exceeded the grasp!