How to Mint 19 Cinnamon taskbar application remove grouping
Right click on the panel, go to "+ Add applets to the panel". The applet, "Grouped window list" should be removed, it is the one that adds the grouping that annoys both of us. You then need to add the applet, "Window list" to get the old way back.
FWIW when I did this it worked the old way but the spacing was very broken. Between the menu and the window list there was a big gap, I reduced it by adding the "spacer" applet, but only somewhat. The quicklaunch icons are to the left of the window list now too, can't figure out how to fix this one....yet. In any case it is annoying but still far better than the Windows X inspired annoyance.
As of Mint 19.1, this can be done from the
Welcome screen (search 'welcome screen' in the search bar of your menu)
On the left, select
First Steps, then under the
Desktop Layout section in the main window select
Traditional. You'll probably have to re-create any shortcuts you had.