So far, I have left the management of my Lightroom Classic catalog files to Lightroom and have never even considered touching them. Lately, however, I have noticed that some them have last been changed many years ago and have not been altered since then. For example, the file "Lightroom Catalog-2-2-v10-v11 Smart Previews.lrdata" is 61.98GB in size and was last changed in June 2014 (see attached screenshot for reference). My questions:
Does Lightroom Classic (v11.0) still need those files, or can I delete them to save disk space?
If some of the files listed in the screenshot are obsolete with the current LR version, how do I know which ones are still needed?
The Library preview folders can be deleted. LrC will recreate new ones when you start LrC, new ones that is. Generally they are ones with .lrdata at the end (warning the "mobile downloads.lrdata" folder is different, perhaps leave that one alone)
The lrcat-data file is new to v11. contains info on select sky and select subject masking. Do not delete that file
As stated, not so sure about "mobile downloads.lrdata"
Looks like you are looking at the folder while LrC is open. The .shm, the .wal, and the .lock files will be deleted when you close LrC. The .lock file is created upon LrC catalog open to prevent multiple instances of the catalog in use. If it exists before starting LrC, LrC will not open the catalog and LrC will close. Generally no reason to bother with the .shm, the .wal files.
It appears you keep backups on the same drive, bad firm, the drive goes, the catalog goes, and the backup goes. Backups should be on a separate hard drive, preferably multiple.
You have the old v10 catalog still. DO NOT DELETE THAT ONE JUST YET. Too many problems with v11, you might wind up rolling back to v10 for all anyone knows, v10 will not be able to use the v11 catalog. If you trust a backup of the v10 catalog, then maybe delete the old v10 catalog.
While some people will tell you that you can look at the catalog name, or the catalog date, and make a decision, I disagree.
Since no one here across the Internet knows what is in your catalogs, I believe you — and not anybody else — have to look in each catalog with your own eyes, and see what photos are in there, and only then you can decide to delete the catalog, or not. Without knowing what is in your current catalog file, and seeing what is in one of the older catalogs, to make sure the images from the older catalog are in the current catalog, you are just guessing and can make mistakes.
Also, I would NOT go by the dates of previews, again for the same reasons as above. (You can delete the previews from 2014, because previews can always be regenerated, but do not delete the corresponding catalog without further investigation).