Thank you for great instructions!
I'll first mention that I did restart my computer before posting here just to see if that would either cause the "Your current security settings prevent the installation of system extensions" message in System Preferences to disappear or the System Extension Updated dialog window to reappear. Neither of those things happened.
According to the Date Modified column in Finder, none of the following extensions were recently modified:
$ ls -a /Library/Extensions /Library/Apple/System/Library/Extensions
/Library/Apple/System/Library/Extensions:
. AppleKextExcludeList.kext RemoteVirtualInterface.kext
.. AppleMobileDevice.kext
/Library/Extensions:
. HighPointIOP.kext SoftRAID.kext
.. HighPointRR.kext
Also, looking at System Information > Software > Extensions, the following is a list of all the non-Apple extensions (and none of them have been recently updated):
ArcMSR:
Version: 1.4.2
Last Modified: 2021-12-08, 01:39
Bundle ID: com.Areca.ArcMSR
Notarized: No
Loaded: No
Get Info String: Areca RAID Driver 1.4.2
Obtained from: Identified Developer
Kind: Intel
Architectures: x86_64
64-Bit (Intel): Yes
Location: /System/Library/Extensions/ArcMSR.kext
Kext Version: 1.4.2
Loadable: Yes
Dependencies: Satisfied
Signed by: Developer ID Application: Areca Technology Corporation (34JN824YNC), Developer ID Certification Authority, Apple Root CA
HighPointIOP:
Version: 4.4.5
Last Modified: 2021-12-08, 01:39
Bundle ID: com.highpoint-tech.kext.HighPointIOP
Notarized: Yes
Loaded: No
Get Info String: Version: 4.4.5, Copyright (c) 2020 HighPoint Technologies, Inc.
Obtained from: Identified Developer
Kind: Intel
Architectures: x86_64
64-Bit (Intel): Yes
Location: /Library/Extensions/HighPointIOP.kext
Kext Version: 4.4.5
Loadable: Yes
Dependencies: Satisfied
Signed by: Developer ID Application: HighPoint Technologies, Inc (DX6G69M9N2), Developer ID Certification Authority, Apple Root CA
HighPointRR:
Version: 4.22.1
Last Modified: 2021-12-08, 01:39
Bundle ID: com.highpoint-tech.kext.HighPointRR
Notarized: Yes
Loaded: No
Get Info String: Version: 4.22.1, Copyright (c) 2020 HighPoint Technologies, Inc.
Obtained from: Identified Developer
Kind: Intel
Architectures: x86_64
64-Bit (Intel): Yes
Location: /Library/Extensions/HighPointRR.kext
Kext Version: 4.22.1
Loadable: Yes
Dependencies: Satisfied
Signed by: Developer ID Application: HighPoint Technologies, Inc (DX6G69M9N2), Developer ID Certification Authority, Apple Root CA
PromiseSTEX:
Version: 6.2.13
Last Modified: 2021-12-08, 01:39
Bundle ID: com.promise.driver.stex
Notarized: No
Loaded: No
Get Info String: Version: 6.2.13, Copyright (c) 2010-2019 Promise Technology, Inc.
Obtained from: Identified Developer
Kind: Intel
Architectures: x86_64
64-Bit (Intel): Yes
Location: /System/Library/Extensions/PromiseSTEX.kext
Kext Version: 6.2.13
Loadable: Yes
Dependencies: Satisfied
Signed by: Developer ID Application: Promise Technology Mobile Apps (268CCUR4WN), Developer ID Certification Authority, Apple Root CA
SoftRAID:
Version: 6.0
Last Modified: 2021-12-08, 01:39
Bundle ID: com.softraid.driver.SoftRAID
Notarized: Yes
Loaded: No
Get Info String: SoftRAID version 6.0, Copyright © 2002-19 Other World Computing, Inc. All rights reserved.
Obtained from: Identified Developer
Kind: Intel
Architectures: x86_64
64-Bit (Intel): Yes
Location: /Library/Extensions/SoftRAID.kext
Kext Version: 6.0
Loadable: Yes
Dependencies: Satisfied
Signed by: Developer ID Application: Other World Computing (Q9P8K45M5C), Developer ID Certification Authority, Apple Root CA
I followed your instructions to change the Security Policy to Reduced Security (Allow user management of kernel extensions...), restarted the computer, logged in, and waited maybe 20 minutes. System Preferences no longer showed the "Your current security settings prevent the installation of system extensions" message, and there was no dialog window or anything popping up. I then changed the Security Policy back to Full Security, and still the System Preferences shows no message about system extensions.
The cause of all this remains a mystery. But at least the nagging System Preferences message is gone.