How can I protect my iPhone passcode if I have to enter it in a public place?
If a thief steals both your iPhone and passcode they can use your iPhone and passcode to make ApplePay purchases and they might even change your Apple password and lock you out of your account and then continue to use your iPhone as if they were you and have access to your stored passwords. Apple's Stolen Device Protection feature greatly enhances protection against some of the risks and you should consider turning this on if you haven't already done so. Significant risks will remain and you still need to guard your passcode from discovery. The probability of a thief stealing both your iPhone and your passcode may be low, but I think it's advisable to take great care whenever you have to enter your passcode in a public place. A likely situation when you need to enter a passcode is when you are trying to pay using Apple Pay and TouchID or FaceID fails. VoiceOver users have some good options. You can turn the rotor to handwriting and then handwrite the passcode, which doesn't display the code on screen or speak the digits or letters as they are entered. Alternatively, if you are using headphones or bluetooth hearing aids, you could turn on the screen curtain with a three finger triple tap and then enter your passcode as usual. The screen will be blank so there is no chance for anyone to observe the code being entered and while VoiceOver speaks the pascode as you enter it, that will be heard only by you.
If you aren't using VoiceOver the best you can do is to try to protect your screen from prying eyes. If you find yourself entering your passcode in public frequently then it may even be worthwhile to learn enough VoiceOver so that you can turn it on and use it to enter the passcode and then turn it off again.
If you use Apple Pay and frequently have problems with TouchID or FaceID then another alternative would be to acquire an Apple Watch and use this for Apple Pay instead of your iPhone. Apple Watch has excellent and simple VoiceOver for those who need it and Apple Pay is always immediately available with a double click of the side button on the watch. The watch doesn't need to ask for your passcode when you pay; it asks for your passcode every time you place the watch on your wrist and so long as the watch remains on your wrist it is ready for Apple Pay. You could use the Find Devices app on the watch to locate and mark your iPhone as stolen, too!