6 months ago
I recently upgrade from Linux Mint 19.3 Cinnamon 64-bit to Linux Mint 20.2 Cinnamon 64-bit. It appeared that a few folders in my home directory I kept were still wearing emblems that are not available anymore in the new version, using right-click on folder then properties. So it was impossible through the graphic user interface to remove these old emblems.
But I found a solution, using a command-line app named gio.
Using this program within a terminal, it's possible to list all attributes of a folder:
gio info ~/path-to-folder
At the end of this list, metadata attributes and their values for this folder are shown. gio can then be used to set another value to an attribute of a folder. So the old emblem can be removed just by using:
gio set ~/path-to-folder metadata::emblems 
where obviously  ([ immediately followed by ]) indicates no emblem.
Once the command is issued, refresh a view showing the folder and its old emblem is gone :)
If you want to keep your old emblems (and I do), or indeed get some extra emblems, create a new folder ~/.icons
All emblems appear to be prefixed with 'emblem-' and appear to be .png files (I'm not sure if the png thing is essential).