well, sitetree wasn't designed for that usecase. Every branch of the tree is a part of an url. You might create a special page type that doesn't append the URLSegment, but you also need to modify ModelAsController to ingore that part if it's that page type.
I did it once vice versa. I have a "AccommodationHolder" with a lot of "AccommodationPages" and wanted to inject the town of each accommodation. It took some time until it worked - and it's not very elegant code.
but with xdebug and a bit of digging around you can surely inject some kind of "FolderPages". I'd suggest the customer to use SiteTree's table view and search functionality, in combinatino with Lumberjack for Holder/Page patterns
Why do these nesting levels exist?
@sorokin At you, wondering why they have the nesting levels if they don't want the URL that way
I don't know, i guess they just want to organize the pages into groups. Their reasoning isn't part of the change request I got.
Hmmm, GroupedPages sounds like a viable module actually 😉
Although, if you look at it very technically, all children of a page are already a group... but it adds the URL segment