User Choice Protection Driver (UCPD) may be blocking the change at the kernel level. You can try adding these PowerShell commands under "Run custom scripts" / SYSTEM: Code: Set-Service -Name UCPD -StartupType Disabled; Disable-ScheduledTask -TaskName "\Microsoft\Windows\AppxDeploymentClient\UCPD velocity" This will disable UCPD, and allow you to make more HKCU changes related to taskbar which are protected by the kernel driver.
Code: Reg.exe add "HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Search" /v "SearchboxTaskbarMode" /t REG_DWORD /d "0" /f Reg.exe add "HKLM\SOFTWARE\Policies\Microsoft\Windows\Windows Search" /v "SearchOnTaskbarMode" /t REG_DWORD /d "0" /f These keys work at the specialized stage. Tested on Core 26100.4061.
set SearchboxTaskbarMode regkey with reg.exe still works here. it works directly in regedit too not protected by UCPD For UCPD details see https://forums.mydigitallife.net/threads/taskbarda-widgets-registry-change-is-now-blocked.88547/
Thanks for the reply, following your pointers, I changed the unattended configuration and now the search box is finally hidden.
Thank you for letting me know about the previous discussion on UCPD issues and the solutions people have used to solve them.