Hello,
I'm in a next gen project, currently trial in standard project plan.
As an admin or a more important role it makes sense to be able to see some of the side bar options, like the "Deploy" , "Board", "Sprint", "Code" or shortlinks features ( that are controllable in the project features section), but I don't feel other users with the viewer / collaborator, or some customized roles, should see it!

Preferable be able to restrict the top navigation menu would be a plus. Our cases really just need access to be able to comment on issues, and edit some fields, and see dashboard, not much else.