The closest Ably alternatives are TalkJS, Stream and Sendbird for chat with a ready-made UI, PubNub and Pusher for hosted real-time messaging, Socket.IO to run your own server, and Firestore for synced data.
Migrate from the shut-down Pusher Chatkit to TalkJS with this step-by-step guide, covering data export and import and rebuilding your chat UI with TalkJS themes.