I believe some programs used to let you even drag menu items to the toolbar.

Many KDE apps (Dolphin, Kate, Okular, etc.) let you configure their tool bars (or get rid of them entirely) and set them to show just icons, text, or both (with the text to the side or below). It's the kind of thing most people won't bother with, but for frequently used applications it's nice to be able to customize it to suit your needs. It's done via a config option though, not by dragging menu items to the toolbar (which strikes me as something you could initiate by mistake).

MS Office’s fully customisable toolbars, complete with built-in icon editor.

…ripped out when the Office Ribbon was introduced in 2007; the now-limited customisation is now considered an improvement because of the IT support problems caused by users messing up their own toolbars.

I mean, yes; but that’s what Group Policy is for! And the removal of the icon editor is just being downright mean to bored school kids.

You made me feel old by saying "I believe".