Thank you for your response. I haven’t explained things very clearly so forgive me.
This is an ordinary Bookmark Search.
I am talking about right-click on the search and getting "Show in Collections".
For instance, in the screenshot I am looking for "MisterArt.com" (Red Oval) but I've forgotten that particular website.
I know it's a Folder with "Judy Greene Music" (Green Oval) but I have forgotten the Folder it's in (in this case, it's in "Score Paper" Folder in "Purple Oval").
I used to have Bookmarks open, press Cmmd + "F", search for "Judy Greene Music" and be able to right-click on the search and use "Show in Collections" and it would find the exact Bookmark "Judy Greene Music" in the Folder (of which I have hundreds) called "Score Paper" so I could find "MisterArt.com".
If you go to the Bookmark menu, and select Edit Bookmarks, you get a similar view to the old bookmark window. Command-F does allow you to search. You won't see the usual search bar, but typing filters the results as you'd expect. This seems buggy and may get removed (or fixed) in a future update, but for right now it more or less works.