Understanding lock file lightroom classic

Good to hear. This is definitely an improvement!

"Users who get the error message (Lightroom can not open this Catalog because it is open in another app), keep telling me that they can just restart the computer to reopen the "locked" catalog file."

If they're running LR 10.1.1, I don't know why they would ever see the warning dialog after a crash, unless the LR process is still running but no longer has a window and is not responding (a "zombie" process, something I've seen just a few reports about over the years).  Perhaps the change was relatively recent and they're running an earlier version of LR.

Restarting Windows will close all existing processes and the "PID counter" gets reset. So after a restart it's very unlikely that any PID recorded in the .lock file will match that of a currently running user process.

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Being a nerd, I did some experiments to determine the behavior of the catalog .lock file with LR 10.1.1 on Mac OS 10.15.7 Windows 10.  The behavior on the two systems is almost identical and has been improved in some recent version to avoid needless warnings.  Here's how it works:

When LR opens a catalog, it creates the .lock file, which contains the path to the LR app and its current process id (PID). When LR exits normally, it deletes the .lock file.

On Mac, the .lock file contains:

path-to-app

PID

On Windows, the .lock file contains

<unknown>

PID

(The literal string "<unknown>".)  

When opening a catalog, if a previous .lock file exists, LR reads its contents and checks if refers to another process that's  currently running.  If it finds such a process, it opens the warning dialog. If it doesn't find such a process, then it reasonably assumes that the .lock was left around due to a program crash and proceeds normally, without the warning dialog.

The precise check differs slightly between the two platforms. On Mac, it sees if there is a process whose id is PID and whose path path-to-app. On Windows, it sees if there is a user process whose id is PID.   

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Yes, I have noticed too that I can restart Lightroom on my Mac after a crash, without looking for the .lock file first. I kind of like it, I must admit (although Lightroom rarely crashes). I don't know if this is a bug, or if Adobe realised that the lock file does more harm than good if it is present when you start Lightroom. It usually results in panic, because most Lightroom users don't know what happened and what to do. And let's be practical: What is the chance that another application has really opened the catalog already? What other application do you have on your Mac that can open the catalog file in the first place? Apart from perhaps beta-testers, who may have two versions of Lightroom that can both open the same catalog, there will be very few people who really need the lock file to protect them against catalog corruption.

"a bug or a feature?"  Beyond my knowledge, sorry. We will have to wait for a more knowledgable person to reply.

If a bug- lets hope it is fixed.

If a Feature (Opening LRCAT with 'LOCK' present)- I cannot imagine why, as it is designed to avoid catalog corruption. Who would NOT want the 'lock' to work as it should?

I have seen reports of users re-opening with the .LOCK file in existence, but I think, to be authorative in your "help to LrC users" it might be best to stick to the official 'Help' article by Adobe. (Not updated since July 2019!)

https://helpx.adobe.com/lightroom-classic/kb/lightroom-error-catalog-cannot-be-opened-lrcat-lock.html