I think it's called anecdotal evidence.
There used to be more pictures when they've been in a city together especially Stockholm. Joel has just been in Stockholm, didn't meet with Matias. He's living in LA yet no sightings of him with Gustaf right now even though he's shooting there. They also don't follow each other on IG anymore which they used to do.
I don't know about the iced out as I think he is still friendly with other people from the Swedish Mafia who are friends with the Skarsgårds. And as we saw he congratulated Alex.
Obviously we'll never get crystal clear proof since I doubt Joel will write a tell-all book about the time he and his best friends broke up. But it's still fun to gossip (see thread title) about the things that point in that direction.
Gustaf has a private Instagram so...maybe that could be it. Maybe they do things, as all celebrities, without us knowing about it.