There’s a lot of Overwatch lore, and not all of it can be easily found in the same place by the casual observer. There are the comics, the short cinematics, and in-game events like Uprising. You might think that the in lobby interactions, where characters chat back and forth, would also fit into this category. The answer is... kind of? Overwatch’s lobby interactions fit into a weird category. Often, it’s what characters would say to each other if they did meet.
Let’s take a look at McCree and Reaper’s lobby interactions.
Reaper: I didn't teach you all my tricks.
McCree: Lucky for me I still got a few tricks of my own.
Reaper: I taught you everything you know.
McCree: Not everythin'.
McCree: You weren't given those guns to toss 'em away like trash.
Reaper: I don't take lessons from you.
Reaper: You look ridiculous.
McCree: Looked in a mirror lately?
There’s some great lore here! We know that Reyes was McCree’s mentor, that McCree has been on his own for a while, and that they have a bitter dynamic based off their past. These short interactions tell us everything we need to know about how McCree and Reaper feel about each other, and hint at future stories to come.
There’s just one problem: McCree and Reaper haven’t met in the current timeline of Overwatch. In a Reddit AMA, someone asked lead writer Michael Chu if McCree knows that Reyes has returned as Reaper. Chu answered:
To clarify about the Reaper/Reyes thing, yes it is because they have not met in the ‘current day’ of Overwatch. The interactions in the game are meant to be what ifs. I do think that McCree would known that Reaper was Reyes if he encountered him.
In short, McCree doesn’t know that Reaper is Reyes, but if the two encountered each other in the Schordinger’s box of a game lobby, he’d recognize him immediately. Meanwhile, Tracer doesn’t know that. She mocks Reaper by imitating his “Death comes!” line when she kills him, but she doesn’t seem to know he’s Reyes at all.
Canonically, the only people who know about Reyes being Reaper seem to be Soldier 76, Ana, and Talon.
Once you realize that the comics, cinematics, and so on are the real sources of canon and the lobby interactions are ‘what if’s, things get a lot easier to decode. For instance, it can be really awkward if Ana is standing there for this interaction:
Soldier: 76: Your mother would've been proud of you.
Pharah: You didn't know my mother very well, then.
or this one:
Mercy: Pharah, your mother always hoped you'd follow in her footsteps.
Pharah: She did? Funny she never mentioned that to me.
or even this one.
Winston: Pharah. Your mother was a hero to me. To all of us.
Pharah: You probably knew her better than me.
Especially if it plays after one of these:
Ana: I'll be watching your back out there, Fareeha.
Pharah: Then I have nothing to worry about.
Pharah: Mum, I know why you didn't want me to join Overwatch; but it's still what I want.
Ana: I didn't want that life for you, but I know that it's your decision, and I will support it.
Those are massively different stories, and they make a lot more sense if you think of what the characters would say if they met in different times. None of those interactions are “canon”, but they’re all shades of the truth. The actual canon is a lot clearer. If you watch Ana’s origins video, you know she’s written to her daughter to let her know she’s alive.
But in Reflections, the Christmas comic, they’re not exactly getting together for ginger snaps and hot chocolate. Pharah is grabbing diner with her father while the hockey game is on, and Ana is dozing off in the Necropolis as Soldier 76 stares at old photographs.
So which lobby interactions are “canon”? We can expect that if characters have met in comics, cinematics, or the “modern day” media of Overwatch, their lobby dialogue is probably canon. Ana and Soldier 76 or the Talon agents probably have these conversations during their current day missions. Characters who have nothing to do with each other, however, are probably exploring a ‘what if’ scenario.