Blocks should only be able to be transformed into other allowed blocks.

[jasonbahl]

Issue Overview

If you define a list of "allowed_blocks" via this filter: https://github.com/WordPress/gutenberg/blob/a1055928b76718dc363be410ad6b2f00ff367c2c/docs/extensibility/extending-blocks.md#hiding-blocks-from-the-inserter

The blocks are filtered out of the inserter, but the "Transform Into" option allows you to transform into any block still.

I think the expectation would be for the transform into option to also respect just the allowed blocks.

Steps to Reproduce (for bugs)

1. Use the filter allowed_block_types to just filter support for a single block type:

add_filter( 'allowed_block_types', function( $allowed_block_types, $post ) {
	return [ 'core/paragraph' ];
}, 10, 2 );

2. Add a paragraph block
3. Click "More Options" on the block
4. See other "non-allowed" blocks you can transform into

Expected Behavior

1. Use the filter allowed_block_types to just filter support for a single block type:

add_filter( 'allowed_block_types', function( $allowed_block_types, $post ) {
	return [ 'core/paragraph' ];
}, 10, 2 );

2. Add a paragraph block
3. Click "More Options" on the block
4. Only see blocks that are "allowed", if any

Todos

• Tests
• Documentation

[jasonbahl]

Any chance this existing PR (#4097) already addresses this? Seems semi-related.

[noisysocks]

canInsertBlockType( state, blockType, parentUID ) which is in #6753 could be useful for addressing this.