“Notion style” is an informal way people may describe a calm, document-like workspace: simple pages, a sidebar, light dividers, and content that feels easy to organize. It is not a formal design standard with one official set of rules.
Imagine a tidy notebook on your computer, where pages and lists help you find information without a lot of decoration. A product team might borrow some of that quiet, organized feeling for its own workspace.
Copying a visual style alone does not make a product easy to use. The layout still needs to fit the task, work on different screens, and remain clear for the people using it.