The data for recipes and furnishing plans are scanned by the addon every patch.

For motifs, part of that dataset is scanned. And part of it is curated. The curated data is the motif association data.

So the automatic scan will discover that there is a Thorn Legion Chests chapter. And the curated association data associates that chapter page with the Thorn Legion crafting style (and also identifies the motif number). The curated data is hard-coded in and is generated by a separate data curator addon. The reason this data curation happens externally is because it involves examining the name of the item, and is thus the process of generating the association data is a language-specific one. However, once the motif item IDs are associated with the relevant style IDs, then that association of numeric IDs is language-agnostic and can be used in any region.

With just the automatically-scanned data (if the curated association data is missing), it's possible to track who knows a particular chapter page or book when looking at that chapter page or book. The association data enables the fancier features like seeing if you know the legs chapter while you're looking at the tooltip for the chests chapter.

All of this happens in LibCharacterKnowledge, which is the core of the addon. Character Knowledge itself is just the the UI.

(If ZOS were to ever add API functions that associates a motif with its corresponding style, then this whole curated data thing won't be necessary. But such a thing does not exist, which is why the curated association data is necessary.)

It is rare, but possible, yes.

Uh.... this feature is enabled by default and has been a part of the addon since the beginning.

No.

I was very specific in how I worded the changelog: The item in question had been removed from the loot table. Not removed from the game.

I, too, am able to craft both and a number of my characters know both. However...

• Although the items have different vendor values and slightly different descriptions (one is a "standard" item and the other is a "small" item), they are 100% identical in appearance and size when placed in a house.
• It can no longer be acquired:

If you've learned the plan from before its removal, great. I have it too, and CK's tooltip feature will still continue support it; the removal affects only the listing in the browser interface. But for the purposes of managing your collection of plan knowledge, it's not helpful to list a plan that cannot be acquired and is effectively a confusing duplicate of another plan.