For quite a few years, Google has provided mobile developers with multiplayer APIs as part of Play Games Services. These can then be deployed in a variety of games. Moreover, a couple of years ago, the Mountain View giant ended support for Play Games Services on iOS, and slashed some features in the Android version.
Now, Google has announced that support for real-time and turn-based multiplayer APIs will be ending on March 31, 2020 (via 9to5Google). This means that, from now onwards, developers won't be able to utilize the aforementioned APIs for new games.
As to existing apps already using real-time and turn-based APIs, developers have until the deadline to switch to alternative multiplayer services. The ones recommended by Google as possible options are Firebase Realtime Database and Google Cloud Open Match. The former does offer a free tier, while the latter is open source. In comparison, however, all fees associated with Play Games Services were included with Google Play developer account fees, so some developers may feel the difference in that respect if they go for a paid tier.
Notably, sign-in, achievements, leaderboards, cloud saves, and other multiplayer services will continue to be supported as they are, with the change next year not affecting them in any way.