After installing Ubuntu MATE 20.04 on a MacBook I was perusing the "Add to Panel" selections and decided to check out "Global Application Menu" (description: "All menus will hosts here). Alas, since there were already so many cool items in the Panel, having a giant Bold font naming whatever app is on top didn't work well. Among other things, the app names disappear under other icons.
The normal method of 2-finger trackpad touch (equivalent to a mouse right-click) on the app name didn't pull up the menu with a "Remove From Panel" option, and no other key combo or anything else I tried would work either. Plugging in a mouse didn't work either. Googling it brings a flood of suggestions, most looking complex and unpleasant with reports they no longer work anyway.
Then I used another option in that Panel menu: "New Panel" to make a fresh empty one where I could experiment further. That's when I saw it: a small vertical column of three dots just to the left of the giant current app name. Because my original panel was a custom color with semi-transparent background the dots were completely invisible. On the default gray second panel they're just barely visible.
Long story short: right-click (or on a MacBook 2-finger touch) on the dots if you can see them. If not, just do that on the left edge of the app name. If the app menu comes up, click again to turn that menu off and keep moving the mouse pointer a tiny bit more to the left until the "Remove..." option comes up. If you go too far to the left you'll either see the Remove menu or the one for the next item. I don't know about other MATE versions, but this works like a speeding silver bullet on 20.04 - including on my desktop PC.
Since so many people seem to be searching for ways to do this, my hope is some will find this post and spare the hours I've just invested in this little niggling problem. 