The second change came to light just recently. It appears that Microsoft disabled the Registry key DisableAntiSpyware which administrators could use to disable Microsoft Windows Defender.
Our colleagues over at Deskmodder note that third-party software such as Defender Control should still work. The equally excellent Configure Defender may work as well.
It is unclear at this point in time if the Group Policy options to disable Windows Defender still work.
The August 2020 update for Microsoft Defender Antivirus introduced a change to the program that made it ignore a Registry key that administrators used to disable the security software.