In Classic projects, there is a "Browse Projects" permission. This makes it possible to give jira-users read access to one project, without allowing them to see all the other Jira projects.
This is not possible with Next-Gen. If a jira-user has any permission in a Next-Gen project, they can automatically see the full Jira project list. This is an issue if the jira-users include clients, for example, who have privacy concerns.