Preventing accidental deleting of images in Lightroom Classic

Have you considered setting up a Smart Folder that collects either rated (e.g. 5-stars) or keyworded (color coded or text, even combined with keystroke command), that copies the original into another different folder that you maintain solely for archival?  You'd lose some disk space in duplication but they could be on separate volumes and it'd give you more long term control anyway. 

The simple answer is no. However, this is no problem because you should back up your images, and back up the Lightroom catalogue. If you ever make any kind of mistake, you can then recover your pictures and any work you've done on them.

Being able to lock images is a fairly popular feature request:

https://feedback.photoshop.com/conversations/lightroom-classic/lightroom-ability-to-lock-photos-please-protect-me-from-myself/5f5f44f24b561a3d422ae94b

Please add your constructive opinion to this feature requestand be sure to click Like and Follow at the bottom of the first post.  Even though this request is longstanding, Adobe sometimes does act on older requests, and adding your and opinion will make it a little more likely that Adobe will consider implementing the feature and you'll be notified when they do. Product developers rarely participate in this forum and won't see your feedback.

In view of your further explanation I'll revisit my first answer. Your issue is not really accidental deletion, it is deleting an image intentionally, when unaware that there was a reason to keep it - such as membership in a particular valued set of images. So that is a matter of labelling the individual image somehow, to make you aware, or of making use of this set of images to do the same thing. A simple "do not delete me" status would not be informative about WHY something should not be deleted.

But a colour flag or a keyword, denoting e.g. that a given image participates in a Book project, can be informative. Alternatively if images for each Book project are gathered into a dedicated collection for the purpose, perhaps that collection's name could include the word "book".  

What I have in mind is something like this:

Use X key ("reject") to flag some candidates for potential deletion. Either use the standard LrC feature of reviewing rejected images, or use a Smart Collection, to present these images together. Then un-reject any images that show a reason for being kept - as above. Then those that are still rejected are approved safe to delete. 

Alternatively a special Smart Collection can select only for those images that do have a Reject flag, but that also fail to meet a series of tests on whether they should be preserved nonetheless.

In other words this SC inclusion criteria might look something like:

must meet  ALL of -

does show Reject flag; is not a member of any collection that includes the word "book"; does not have a green colour flag; lacks a certain keyword; whatever.

The consequent list of images in the SC can be safely highlighted en-masse and deleted, knowing that nothing worthy of preservation will be included. Other images that you do want preserved will still show their Reject flag, but will have been spared from deletion.

As dj_paige says, you can store the image source file in a folder which has read-only permissions (for the particular user account you are using) but this will then impair your ability to write external metadata to the file and also your ability to generate new derived files into the same folder (e.g. with Edit in Photoshop). An intermediate position may be to mark a particular important Raw file as read-only at the file level, which will then block deletion of that individual file - while permitting continued updating of XMP sidecar as well as creation of derived files alongside. This seems laborious to me, better to just concentrate on good file backup.

So far as being imported to the Catalog, there is no way to block removal of an image instance - however if you make use of virtual copies then that does offer some protection in that deleting one of several image versions referring to a given imported original, retains those other versions still in place. When you work on an image in the context of a Collection, pressing Delete in error does not risk removing the image entirely from the Catalog - it will only withdraw its membership in that particular Collection. So it can be added back into this Collection once you have found it again, e.g. in its folder, with nothing otherwise lost.

Also if you have LrC set to automatically write changes to external XMP as you work, then in the event that you do remove an image from the Catalog, then presuming the source file and external XMP are still there on disk / if deleted, that these can be retrieved back onto disk from regular backup - then on reimporting this image, that previous editing saved in the external XMP is read back in. You will only have lost Collection memberships, virtual copies and a few other such issues.

A Catalog backup offers a separate way to recover fully from accidental deletion / removal, given that the source file is there or still retrievable. External XMP can offer a separate opportunity, to then bring back in even image edits that happened subsequent to your latest available Catalog backup. Or, to recall the last written-out editing state for any images that were only imported subsequent to that Catalog backup, on importing those afresh. 

You can't do this in Lightroom Classic. Lightroom Classic presents the usual (that most programs have) dialog box cuasing you to take another action (click a buttom) if you really want to delete the photo. But if you do accidentally delete a photo, then Ctrl-Z undoes the deletion. So you have plenty of protection.

You can make regular and automated backups of your photos and of your catalog file, so if you later discover something has been deleted, then you can go back. So you have plenty of protection. (In fact, you should be doing this for all of your important work and documents, and if you're not, then you need to start immediately, no excuses, no delaying)

In your operating system, you can change the folder(s) to have only read permission (not write permission) and then you can't delete the photo at all (but you could still remove it from Lightroom Classic).