Google Play Games Services, the backend behind Google Play Games, is paring things down a bit, according to a recent post to the Android Developers Blog, and that includes, among other things, ending support for iOS. New signups for the iOS version of the service had already been disabled back in December of 2016, but Google is now going whole hog and ending support for the iOS version of the SDK and all related features. As far as cross-platform features, Requests, Quests, and Gifts are all officially meeting their end in about a year's time, meaning that developers who want to use features of that nature will have to create their own implementation of them.
The end of iOS support means developers will no longer be able to implement features from Google Play Games Services, and there never was an official, all-in-one Google Play Games app for iOS, which means that the service has pulled out of the platform completely. The native version of the SDK, which can be ported across web games and Android games, will no longer support iOS implementation, and presumably, trying to shoehorn a port of these features into an iOS game won't end well. Many developers had already begun using Requests, Quests, and Gifts, among other features, in their iOS games; those who have will have about a year to remove and replace them. Those services, according to Google's blog post, are officially set to go dark as of March 31, 2018.
To briefly go over the cross-platform features that all developers will have to remove before the date above, Quests allowed developers to drop brand new time-sensitive objectives and rewards for them without the user having to download an update. Gifts, meanwhile, does exactly what it says on the tin; it allows users to send in-game gift items to other users. Finally, Requests tied into Gifts by allowing players to ask other players for in-game Gifts. According to Google, they are only ending support for iOS and the features listed above due to a lack of popular demand and developer utilization. Their post goes on to say that they will continue to work on Google Play Games Services.