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?

Looks like installer plugins are working as expected on beta 4 of macOS Big Sur

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