Google has been working on a new beta version of Pay since November, but the old app has been available on the Play Store for users who aren't ready to transition. At least it was until today: anyone searching the Play Store now will be greeted with only the new app, which is out of testing. The old version is still functional, but should start to disappear as users update.
Google's revamped implementation of Pay focuses less on the mundanity of NFC card payments and more on payment transfers between users, as in roommates paying utilities or a shared dinner bill. It also has some surprising cash back rewards for some purchases, as well as the ability to scan your Gmail and Google Photos accounts for receipt data. It's not all improvements, though: as a part of the transition to the new system, users can no longer send or receive payments on the web.
For the moment you can still find the older version of Google Pay on the Play Store via a direct link. It's even listed as "Google Pay (old app)" in the description. Naturally, both the old version and the new one are on APK Mirror. For the time being the old app is still working, but it's likely that Google will begin to force users onto the new system before too long.