Answer: Microsoft bought it.
Microsoft rep: “I believe you are referring to Teams.”
Sometimes a product winds up in late-stage enshittification before its sector is fully developed. Just as the thing it does is beginning to fully explode, it begins to aggressively harvest its brand value. They miss the big wave, and everyone asks what happened. That’s one disadvantage in moving early. You also hit enshittification early.
Microsoft.
My opinion is that Skype was killed by the smartphone. I used Skype from probably 2003-ish to 2008 as my only source of a home phone number to interact with people who had a phone number, as I did not have or want a cell phone or a landline. After that, I got a cell phone that people could call me on, replacing my need for skype a s aphone number source, and then various video chat and voice calling apps came along completely replacing the functionality I was using Skype for. Then when Microsoft bought it, that put a very sour taste in my mouth at the time, and there was no looking back.
Having centralized authentication servers. Otherwise MS purchase of it wouldn’t kill the normal Skype, and by now it’d be probably, like ship of Theseus, actualized for modern security and other requirements. Like X-Wing Alliance and WoW and Warcraft III modding, and other iconic games getting even replacement engines.
Hotline got FOSS clients at some point, some still work, some people even make new ones.
It was hogging port 80.