How to Unlock iOS 12 with passcode, without disabling touch ID
I use the following technique to present the passcode screen, without disabling the Touch ID:


Press the Home button slowly (once, or if needed, twice, i.e. not fast enough to trigger the Apple Pay screen) with a fingernail or the tip of any of your finger or thumb vertically, such that the Touch ID sensor doesn't register any fingerprint.
This should present the passcode screen without disabling the Touch ID.