Block API: Can't restrict where a block may be inserted using parent

[noisysocks]

Describe the bug

Third parties are no longer able to make it so that a child block C can only be added to specific parent block P while also allowing P to have non-C blocks added to it.

To reproduce

1. Paste the following snippet into the DevTools console:

   Snippet

   ( function() {
   	const { registerBlockType } = wp.blocks;
   	const { createElement: el } = wp.element;
   	const { InnerBlocks } = wp.blockEditor;
   
   	registerBlockType( 'acme/product', {
   		title: 'Product',
   		icon: 'carrot',
   		category: 'common',
   
   		edit() {
   			return el( 'div', { className: 'product', style: { outline: '1px solid gray', padding: 5 } },
   				// Only paragraphs, images, and Add to Cart blocks:
   				el( InnerBlocks, { allowedBlocks: [ 'core/paragraph', 'core/image' ] } )
   				// Everything and Add to Cart blocks:
   				//el( InnerBlocks )
   			);
   		},
   
   		save() {
   			return el( 'div', { className: 'product', style: { outline: '1px solid gray', padding: 5 } },
   				el( InnerBlocks.Content )
   			);
   		},
   	} );
   
   	registerBlockType( 'acme/buy-button', {
   		title: 'Buy Button',
   		icon: 'cart',
   		category: 'common',
   
   		// Only allow in products:
   		parent: [ 'acme/product' ],
   
   		edit() {
   			return el(
   				'button',
   				{
   					className: 'buy-button',
   					style: { display: 'block', margin: '0 auto', padding: '10px 30px' },
   				},
   				'Buy Now'
   			);
   		},
   
   		save() {
   			return el(
   				'button',
   				{
   					className: 'buy-button',
   					style: { display: 'block', margin: '0 auto', padding: '10px 30px' },
   				},
   				'Buy Now'
   			);
   		},
   	} );
   } )();

2. Add a Product block to the post.
3. Select the Product block and open the Inserter.

Expected result

The Paragraph, Image and Add to Cart blocks should appear in the Inserter.

Actual result

Only the Add to Cart block appears in the Inserter.

Additional context

This functionality was introduced in #6753.