Google has updated its support pages and started sending out emails to alert users of Google Wallet to an upcoming change in the way NFC payments work. As of April 14th, tap and pay will require KitKat or higher. Older devices will no longer be supported after that date.
The reason for the change is Google's desire to only use Host Card Emulation (HCE) to make NFC payments work. That feature was introduced in Android 4.4, so it's the end of the line for Jelly Bean and earlier devices.