Excess files in LR library

another perspective: let's define the issue, if that even is a problematic issue (which is not certain) as - there are some files imported to LrC and other files not imported to LrC, all mixed together in the same storage area.

Assuming one wanted to physically separate out just those files managed by LrC, from the rest: that is IMO understandable. It makes some sense to behave differently towards those two classes of files - in terms of backup, in terms of preserving the integrity of the former class by actively leaving them be; in terms of being safe to clean up the latter class without unknowingly impacting the catalog.

LrC can help you do this separation  requiring enough spare disk space into which all the imported images can be selectively copied. In summary:

Export as Catalog into this spare disk space, checking "include negatives" and unchecking "selected images only". This creates a new duplicate Catalog, referring to copies of all the relevant files, that are made by LrC into a subfolder arrangement (to match that seen in the starting Catalog) inside this same spare disk space.

Several ways to proceed from there: among which are, that the originals of all these files that have just been copied can be selectively deleted from disk using this same starting Catalog to discriminate these. Now the starting area of disk contains only files that are NOT imported; while the new area of disk contains only files that ARE imported, with a Catalog alongside referring to those, which you can then go forward with (and stop using the prior Catalog).  Alternatively the starting Catalog can be reverted to a recent backup, and you can switch the two sets of files on disk, then proceed with the imported images returned to the same places as before, and with the remainder now sepatated elsewhere.

Personally I would not bother: part of the job of a Catalog is to distinguish in a live manner which image files on disk are currently included in its formal image library, and which are not - as an intentional, ongoing matter.

Say one culls an image out of the Catalog after import but retaining the source file on disk: well, at some future date one may reconsider and decide to "un-cull" this photo. And there it will be.