Lightroom Classic: Does not "mark" locked files

The "lock" isn't really meant for flagging images, being used to tell the camera not to easily allow deletion of the image. It's quite possible that different cameras implement this in different ways, which may not even involve setting any metadata in the image file itself, so Lightroom would have no way of knowing. However, if this is recorded in metadata of images from your camera, you should be able to find out by comparing metadata from a locked image to metadata from an unlocked one. Once you have found if there is some metadata item used to track this then you may be able to filter for it in Lightroom, perhaps needing one of the more advanced filtering plugins to do so.

Filtering 'locked' images once they have been imported into Lightroom is only a small consideration.

Photo Mechanic, Aperture and several other apps have been recognizing 'locked' images as 'flagged' images for years. I doubt it is too difficult to achieve or deploy the supporting code. It is especially handy when trying to selectively import only the cream of the crop when on very strict  deadlines. 

This is something many photojournalists/sports shooters have been requesting of Adobe to implement in Lightroom since the very first public beta, long before Lr v1 was ever introduced. I know, because I was one of those users who has been requesting such a feature for over a decade now ... to no avail.

It's sad that so many users must resort to using outside resources like PM when Adobe could offer a solution that could help a significant number of users ... many of which made it possible for Lightroom to become so popular in the first place.

I have also requested that Adobe add this feature.  With each new update we get amazing tools and controls.  It would seem that displaying a lock would not be asking for much and it would save me the trouble of going into Finder to root out all the locked files.