The password reveal button in Windows 10 login screen is a useful option to verify if you’re typed the password correctly. However, if you want to disable it for more security, here is a registry edit.
Start Regedit.exe and go to:
Create a DWORD (32-bit) value named DisablePasswordReveal. Double-click DisablePasswordReveal and set its value data to 1
This disables the password reveal button.