Custom icons for folders have disappeared and cannot be re-entered!?!

Make sure you are following the instructions, I have Catalina, Mojave, High Sierra, Sierra and El Capitan and I

use the same instructions and they all work.

Create custom icons for files or folders on Mac - Apple Support

https://www.macrumors.com/how-to/customize-file-folder-icons-mac/

Security updates would not have affected this function.

Do you have any third party software that affect the Finders default behaviour.

It should not be a problem to get your customised icons back working.

If the icons you want to use are jpegs, pngs, tiffs or some other recognised form of image

then open them in Preview, click on Edit> Select All and then Click on Edit> Copy.

Now select the folder you want to change and press Command-i to get the info panel up

highlight the small icon top left and press Command-v to paste the copied icon over.

If the replacement icons have the extension .icns then get the info panel up, make sure the preview panel is displayed

and just drag the preview image over to the small icon in the receiver folder info panel.

I am having this same problem. One day they were all gone. I followed the steps above, as I always have to add custom icons to folders, but it doesn't work anymore. When I paste, the folder flashes then returns to default blue. It is very frustrating, as it is much easier to identify a client's folder when they all have their logo on it as opposed to a bunch of plain blue folders.