The old Registry edit to remove the Accessibility button on the login screen no longer works, i9t seems Windows 11 no longer users that key.
The accessibility button is also there to allow users who do not need it normally to be able to log into Windows using the on-screen keyboard, if their keyboard is not working to log into Windows, which was required by many people with the USB keyboard bug that was in the February update this year.
This forum is just a user to user forum, the best option is to open the Feedback Hub app in Windows and suggest to their developers there that they provide a setting to remove the accessibility icon on the start menu, that is where they will see your feedback.