Extend the Block.json specification to be able to restrict blocks to particular templates

[priethor]

What problem does this address?

With the addition of theme blocks such as Archive Title block, Term Description, or Post Categories among others, the inserter offers, both in the post and template editors, a number of blocks that don't work as intended unless they are inserted in specific templates. For instance, an Archive Title block should only be available when editing an archive template.

What is your proposed solution?

Extending the block.json API to either:

• Be able to restrict blocks to specific templates.
• Be able to disable a block by default unless a template specifically enables it.

[gziolo]

This is very related to the previous comment #13693 (comment).

Extend flexibility of parent propert

• If and when Enables Block Grouping/UnGrouping using core/group #14908, there will be precedent for wildcards in block transforms (source), and it certainly seems like an equivalent behavior could be implemented for parent.

We should create a follow-on issue for this. That said, it's not directly mentioned as being possible in this RFC. Which begs the question: Should it be, if we're saying it ought to be possible?

It's also being tracked in #16209 which contains several proposals to enhance the block metadata stored in block.json files.