lost entire images in folder after a single photo deletion

Please provide more information such as Lightroom version, OS, computer configuration 

I can't tell you what happened, but I do want to react to your remark "This one has me so scared I never want to erase an SD card which is crazy.". Yes, that is crazy because it is caused by using the wrong workflow. When you import images, Lightroom Classic has the 'Make a Second Copy to:' option. Use this to make an immediate (temporary) backup of the imported images! Then make a true incremental backup of your image folders a.s.a.p. After that you can delete the second copy and you can safely reformat your SD card. Of course you should also make a catalog backup each time you worked in the catalog. Don't use the default 'Once a week' option!

Your current misfortune, with which I sympathize, reminds me, in part, of an earlier bug in which deleted photos didn't appear in the recycle garbage can (to make them appear, some manipulation was required, which I no longer remember). Is this a resurgence of the same phenomenon? In that case, the hypothesis of a selection and deletion of all the images in the folder that has been evoked would be possible.
Nevertheless, because I've already managed to erase all the images in a folder through carelessness, for some years now I've opted, in addition to a separate monthly physical copy of the Lightroom catalog and all source folders and images, for a backup using Time Machine (for those using MacOS)... a 12TB hard disk is used, which is hardly enough, but it offers additional security.
On the other hand, I've been very cautious about updating MacOS and, very recently, LR, keeping version 13.0 despite a series of malfunctions and bugs that were never corrected.

Best regards from France

My first thought is that you accidentally had selected all the images in the folder, wihtout realizing it, before deleting. But you do have backups of your photos, don't you? Normally that eliminates the need for relying on SD cards.