Open the registry editor and navigate to

Windows 11 search showing Registry Editor as the best match for regedit

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced\Start_ShowClassicMode

Registry Editor open at HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced, adding a new DWORD (32-bit) value

If Start_ShowClassicMode doesn't exist, create a new DWORD

Change DWORD=1 to enable classic Start

Editing the Start_ShowClassicMode DWORD and setting its value data to 1

Logout and log back in to see the old start menu

Windows 11 showing the classic Windows 10 style Start menu with live tiles

Njoy :)