The number of people playing Valheim has dropped pretty significantly in the past month or so. It's nothing to be concerned about, really—lots of people have moved on from the Viking survival game for now, but I'm pretty sure when Valheim's first big update Hearth and Home arrives, they'll swarm back for more. That's my personal plan, at least.
In the meantime, another Viking game is on the brink of an Early Access release. It's called Frozenheim, and while it's a city builder with RTS combat rather than a survival game...