What qualifies a novel for a 'slice of life' tag?
As far as it being a style of writing goes, I don't think it technically could count as one, but an author could certainly have a habit of writing in a slice of life format (slower paced, less focus on plot, etc.) even if they don't mean to be writing a slice of life story. If an author is always going into detail about everyday life and all of the shenanigans associated with it, then I would say it's less of a style and more of them just writing slice of life without necessarily meaning to. I guess my point is that it's either a genre or a component (which could be mistaken as style) of something more.
Anyway, I feel that slice of life follows a character/characters through a period of their life. It doesn't have to detail their daily life or whatever else, but the main theme of the story is how this period of time affected/changed the character/characters.
Say for example, a story where a zombie apocalypse happens, and the story is about how they save the world.
If your focus for that story is on the world and how it is being saved, that's not slice of life.
But if your focus is on how the protagonist grows, and slowly changes throughout the events, and "saving the world" is just a minor plot point within the story to show the character growth, then it is a slice of life.