Based on our record, Slack RTM API seems to be more popular. It has been mentiond 1 time since March 2021. We are tracking product recommendations and mentions on various public social media platforms and blogs. They can help you identify which product is more popular and what people think of it.
Oh, my bad, in that case you probably have to use the Real Time Messaging API for Slack. Source: over 2 years ago
Blockbutler - Real-time transaction notifications for the blockchain
Amio Messaging SDK - Develop for multiple messengers using JavaScript
Notif.me SDK - A Node.js lib to send all your transactional notifications
Telnyx SMS API - Build global text messaging right in your app
Pusher - Pusher is a hosted API for quickly, easily and securely adding scalable realtime functionality via WebSockets to web and mobile apps.
Pragma - The home base for Ethereum developers.