XMPP

Instant Messaging presence-protocol (Open Standards) developed by Jabber project (2004)

i'm not clear on how this overlaps-with or competes-with SIP

What happened to it? (2021)

• there are still client apps, but where you find a server/service?
  • that article mentions GoogleTalk?!? I didn't know that was still accessible (only to 3rd party clients), but as of Jun'2022 it's being shut down completely
• (2021-08-28) A Decade And A Half Of Instability The History Of Google Messaging Apps
• Plenty of triple-A games and gaming platforms use XMPP to provide chat (and more) - we’re talking Fortnite, Eve Online, and so on. The militaries of 30 different nations use XMPP for text chat on the battlefield, too. It’s used most crucially for coordinating MEDEVAC helicopters, but plenty of other things besides. The Belgian Ambulance service have an XMPP-based mobile app. The theme linking many of these use-cases is reliability for mission-critical text chat.
• 90% of messaging companies either use heavily customized version of ejabberd (WhatsApp, APNs - Apple Push Notification servers) or developed custom Erlang servers for more scalability and mobile friendly protocols, either proprietary or standard such as MQTT.
• 2020 comment: speaking as someone who got paid to (among other things) administer both private and public-facing XMPP server networks in the aughts, there are good reasons why XMPP cratered (multiple incompatible groupchat implementations, competing incompatible reference server implementations, a presence model that simply never worked on mobile, and of course XML, XML everywhere) and I would not claim with a straight face that it's a viable replacement for Signal, iMessage, Whatsapp or any other actually-working chat network today.