2.更为安全/有效的方法是直接将开始菜单删除掉。方法：C:\Program Files\Adobe\Adobe Photoshop CC 2018\Required\CEP\extensions里面的com.adobe.ccx.start文件夹删除就可以了。
Thanks for the reply. Unfortunately, both option did not solve the problem. The first option we already tried, and we can confirmed that if the setting was re-enabled again, all the cached image will recover and appear again even if they had been deleted from disk. The second option cause PS display black screen and no response on start up until we recover the com.adobe.ccx.start folder.
The main concern of our company is the new version of PS CC 2018 seems to cache the image somewhere without our knowledge and we cannot take any control of it. Even if we try to delete these cache, PS just hide them from us. These hidden contents are still accessible and recover by Adobe's software because Adobe known where it is, but the user cannot. This cause a great privacy problem if Adobe use these information without user's knowledge.
The option is already set to 'Never Save', but it still save the thumbnail in background. It seems this has no effect.
If the value of 'Recent File List Contains' change from 0 to 20, all the thumbnails will automatically appear on the first page of PS next time even if these images are deleted from disk.
This is a tough problem. As you can see the thumbnails are internal and not user changeable.
This next idea assumes no one can open your files. Maybe you would keep them on an external drive that you disconnect when you finish.
The trick would be to make the layers in your file invisible by clicking off the eyeball.
The information is still there, but will show as a blank thumbnail after you save out.
Save in a format that supports layers, psd, tiff or psb. Never png or jpeg.
Then whether it is the MacOS Finder, Windows Explorer, or the Photoshop Start screen, only a white thumbnail is visible.
Let me know what you think. The idea may need some fine tuning.
The problem is you want to keep your edited files out of the Thumbnail history.
The answer is Export. An exported file will not show up in Recent Files.
So you finish your edits, but before you make the layers invisible, you use File > Export > Quick Export as JPG.
Then you are ask to choose a location. Make a new Folder and label it Confidential Images. You PSD will be flattened and saved as a Jpeg in that folder. You will make sure that in order to examine them, you use a separate jpeg viewer. Not Photoshop, not Bridge.
In fact to prevent an accidental open in Photoshop, have your jpegs associated with another utility or ediitor.
Do not close the PSD until you make the layers invisible so as to have a blank thumbnail written as you save out.
When you have done that and have inspected and encrypted the Jpegs, you can delete your PSDs. Any PSDs you intend to delete should be all be kept in one folder titled, "Final Edits".
This may need fine tuning, like adding "done" to the filename of your saved, but blank PSDs to keep track.
So let's try that. You should see blank PSDs, but not your Exported Jpegs in Recent files.
Thanks for the reply. We've sort out this problem. We found that these cache are stored in the file "C:\Users\UserName\AppData\Roaming\Adobe\Adobe Photoshop CC 2018\Adobe Photoshop CC 2018 Settings\MachinePrefs.psp". All thumbnails are packed in this file. Each thumbnail size is about 5-7kb. By delete the whole recent file list from the menu, the thumbnails have been removed from this file. Just wipe this file is also ok to clear these thumbnails.