We use the concept of future roles (https://mui.com/careers/#next-roles) to help source candidates ahead of time. We have experienced that a good number of people are interested in joining MUI but are not actively looking for a job.
By default only list future roles on our career page as it targets passive-looking candidates.
Candidates going to, e.g. AngelList, would probably be actively looking, and no longer be available once we contact them.
To publish a job offer as “unlisted”, see this screenshot: keep the checkbox unchecked.