InnerBlocks with a template lock, and the editor who wanted it open

A locked layout that guaranteed the design held, and an editor who needed one more column on one page and had to file a ticket.

<InnerBlocks
  template={[
    ['core/heading', { level: 3 }],
    ['core/paragraph', {}],
  ]}
  templateLock={attributes.locked ? 'all' : false}
/>

// and the toggle in the inspector, available to editors
// with the right capability only

all prevents insertion, removal and reordering; insert allows reordering but not adding; false allows everything. Making it an attribute with a capability check turns a design decision into a per-instance one, which is what the situation actually was — the layout should hold by default and there is always a page where it should not.