I’m going to venture to guess that this means they left a particular conversation in PM.
I was just now going through all of my PM’s and cleaning stuff out and I noticed that someone I was speaking with had a line through their name and their av was darkened as well but upon looking in several other PM’s I had with this same person, their name was not lined out nor was their av darkened in the others.
I actually came here to ask this same question but checked because I thought it had been asked before.
So I’m gonna say that it is in fact merely that that person has left the conversation. Just my two cents.