It’s not a shonen, despite the title. I’d argue it’s for more mature audiences. The story is a tragedy about a man who is forced into immortality, and spends his days fighting demons to prepare himself to kill the demon who did this to him. It’s definitely got some action, but it’s not really about the spectacle of fighting demons. Most of the demons are caused by bad emotions like grief, anger, jealously, etc. The story focuses on how these emotions demons affect people and the world around them, and the main character learning more about himself and the world during his journey.
I’m not good at explaining this, just watch it lol.
I’ve watched it up to the latest episode on HiDive and I have been enjoying it. It’s certainly not something I would classify as a Shonen. And it for sure focuses on the idea that you shouldn’t let your emotions take control, lest they become your demons. Which I’m sure is part of the reason the anime isn’t very popular in the west.
Available information suggests that the source material is considered seinen rather than shounen, so it is indeed aimed at an older crowd. Good seinen material tends to be more thoughtful than shounen. (Bad seinen material is just shounen with added hentai, though.)