The official shutdown date for Yahoo Messenger has been decided, as well as a Yahoo-hosted alternative group messaging service.
According to Yahoo's official support site article about the discontinuation of Yahoo Messenger, the currently invite-only app Yahoo Together is being billed as an alternative to the service once it discontinues on July 17th. Squirrel, which is currently in beta, is a more centered on group conversations in a similar vein to services like Discord or Slack, with the ability to create rooms for individual topics.
Outside of Squirrel, Yahoo states that "There currently isn’t a replacement product available for Yahoo Messenger." For applications with similar functionality (that aren't as group focused as Squirrel and its alternatives), see services such as Signal, Telegram, and WhatsApp Messenger.