Lightroom Classic does not "index" folders or files.
It only imports / does not import a particular image file, as you decide. And then the Folders panel shows a summary report of all the file paths belonging to the particular image files that are currently imported.
If it has imported an image file, then it must therefore show that folder. It can also be be made to show a folder that has not got any imported images directly inside, but will never show any images that are not imported.
And if (for example) you create your new ARCHIVE subfolder after importing those images in the main folder, LrC will not now show this subfolder unless either: you tell it to by using "add folder"; or: you import an image which is located in that subfolder.
So the basic selectivity is: which images do you want to import.
And that depends on how new images are arriving in your server (and being organised in folders there). If LrC is in sole charge of that (Copy import with auto filing) then only the folders directly required for this operation will show. If you are first putting the images into the server and then telling LrC to import them (Add import), you can un-check "include subfolders" and click on that main folder to only import the direct contents of that. Also, in the Import screen there is the option in the left panel, to un-check particular subfolders from having their contents included.