NFC payments on Android used to be very much free for all, but the company increasingly clamped down as Android Pay (and now Google Pay) rolled out. When a device doesn't pass SafetyNet because of modifications or even an unlocked bootloader, you can't make NFC payments. That was a problem with the first Android P developer preview, but the newly released DP2 does allow you to use Google Pay. Well, in some instances.
An unlocked bootloader is still a no-go for Google Pay, so it's the launch of the beta program that matters here. If you upgrade a device to Android P DP2 via the OTA, you don't need to unlock your bootloader. Leave it locked, and your phone will pass the SafetyNet check on Android P.
For the previous developer preview, you could use Google Pay if you relocked the bootloader after flashing to your unlocked phone. With DP2, your phone can do payments out of the box after you install via the OTA. I would not, probably, install the DP2 on your daily driver, though. It's pretty rough around the edges.