My main concern is still "how do I deactivate the block pattern directory in a client site which doesn't need public patterns".

Lets say I'm specifying a site for a client and they've requested they want a mostly locked down page template with a few editable or switchable elements. They wish to (Create New) Article and see a WYSIWYG layout with key elements fixed in position, editable but not removable, then to select from 3 potential sub-page page layouts (EG: Magazine / Academic Paper / Podcast ) where they can edit safely. In that area they want their editors to have access to a few blocks relevant to the context.

So... to accomplish this first I need to lock down the page template in general and unlock the sub-page using template_lock nested templates. There is only this documentation for this feature:https://developer.wordpress.org/block-editor/how-to-guides/block-tutorial/nested-blocks-inner-blocks/ then lock the outer blocks and unlock the inner blocks with https://developer.wordpress.org/block-editor/reference-guides/block-api/block-templates/#locking

Next I must hide all patterns which are not provided by me (deregister using remove_theme_support( 'core-block-patterns' );)

Now lastly I'll want to completely prevent public blocks from being used, and hide the upcoming UI for the public block pattern directory to prevent Jimmy The Intern from inserting a non-approved external pattern into the approved layout.

There is no documentation for the pattern directory feature. The UI is not specified yet. The GitHub has no docs, information is scant. I'm very busy and I need a central documentation reference for this massive new feature.

The Directory will launch In July 2021 and might appear in the client site at any point afterwards, perhaps part-way through development! So I'll be needing to deal with it, to turn it off at the very least!
But how? Who knows!

Currently the best option we have seems to be "email Justin". Now that's very kind but is it really the best option?

This is why people like me avoid the Block Patterns. I'm actively looking for a solution for sub-page user-selectable templating .. and yet this Block Pattern/ Directory solution looks a lot like a problem for me. It's another asteroid heading for my planet.