For this, go to: needed viewport → Viewport label menu in left top angle → 2nd item with view name (e.g., perspective) → Show safe frames item; or just press shortcut Shift+F.
Listed all ways to do it:
- Keyboard → press Shift+F shortcut.
- Needed viewport → Viewport label menu → 2nd item with view name → Show safe frames item.
- Pass by any from these ways to open Viewport Configuration window:
…and next go to: Safe Frames tab → Application group → Show Safe Frames in active view checkbox.
- Main menu → Views item → Viewport Configuration item
- Needed viewport → Viewport label (looks like plus sign in square brackets – "[+]") → Viewport label menu → Configure Viewports item
- Scene menu → Configure Views → Viewport Configuration item