Installer plugins not getting loaded on macOS Big Sur

Yes, seeing similar problem. Also finding this in Console: Library Validation failed: Rejecting '/private/tmp/com.apple.installer4tmhYtZ1/myInstallerPlugin.bundle/Contents/MacOS/myInstallerPlugin' (Team ID: ****, platform: no) for process 'InstallerRemoteP(3911)' (Team ID: none, platform: yes), reason: mapping process is a platform binary, but mapped file is not

I notice that Catalina's Installer.app has this output from codesign -dv:
CodeDirectory v=20100 size=551 flags=0x2000(library-validation) hashes=12+3 location=embedded

But Big Sur has:
CodeDirectory v=20100 size=2564 flags=0x0(none) hashes=75+3 location=embedded

Yes, seeing the same problem here:

error 09:29:29.624377-0700 kernel Library Validation failed: Rejecting '/private/tmp/com.apple.installerbBJCPsLO/***.bundle/Contents/MacOS/*' (Team ID: ********, platform: no) for process 'InstallerRemoteP(1890)' (Team ID: none, platform: yes), reason: mapping process is a platform binary, but mapped file is not

Yes i have this issue too. I can see the circle where the Installer Plug-In should be, but it is not shown.
I also opened an issue yesterday about this.
I allready gave the macOS Installer Full Disk Access and also opened the pkg as root, but i still get an error which hints to missing permissions.

Anyone got any clue or a work around for this problem, to get the installer plugins working on Mac OS Big Sur

I think this is the codesign validation issue of Installer.app. Apple should fix it their InstallerRemoteP codesign validation.

Here is what I see with my plugin:
kernel Library Validation failed: Rejecting '/private/tmp/com.apple.installerDMqsdWnZ/MyPlugInName.bundle/Contents/MacOS/MyPlugInName' (Team ID: XXXXXX, platform: no) for process 'InstallerRemoteP(10235)' (Team ID: none, platform: yes), reason: mapping process is a platform binary, but mapped file is not.

I also encountered the same problem and reported the same error.
I can use the "csrutil disable" command to turn off SIP and it can work normally.
Can anyone answer me this is Apple’s feature request?

I can confirm that installer plugins are working again in beta 4.