what is yahoo messenger?

Instant messenger no longer exists. It was closed in July 2018: https://help.yahoo.com/kb/SLN28776.html

It's an instant messaging service that Yahoo has operated for two decades until its closure almost a year and a half ago.

Something that no longer exists.

Yahoo! Chat was a free online chat room service provided exclusively for Yahoo! users. Yahoo! Chat was first launched on January 7, 1997, as it was confirmed to be a feature on the very first release of Yahoo! Pager. On March 9, 1998, the first public version of Yahoo! Pager was released, with Yahoo! Chat among its features. It allowed users to create public chat rooms, send private messages, and use emoticons.  

In June 2005, with no advance warning, Yahoo disabled users' ability to create their own chat rooms. The move came after KPRC-TV in Houston, Texas reported that many of the user-created rooms were geared toward pedophilia. The story prompted several advertisers, including Pepsi and Georgia-Pacific, to pull their ads from Yahoo.
  
On November 30, 2012, Yahoo announced that among other changes that the public chat rooms would be discontinued as of December 14, 2012. quoting "This will enable us to refocus our efforts on modernizing our core Yahoo products experiences and of course, create new ones." 

Yahoo! Messenger dates back to Yahoo! Chat, which was a public chat room service. The actual client, originally called Yahoo! Pager, launched on March 9, 1998. Yahoo! Messenger was shut down entirely on July 17, 2018.

It was a way for people to chat privately or in a group. It was the best form of messenger available to the public and its a shame Verizon removed it.

a long gone messaging system

you used to be able to send messages on there but i think they took that out

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A device that stopped working.