Saw this advice from multiple sources, most pointing to @stroughtonsmith:

defaults write -g NSMenuEnableActionImages -bool NO followed by a computer restart results in the menu icons being hidden, except for when apps specifically overwrite this settings.

It even preserves the couple of instances you do want icons, like for window zoom/resize.

Firefox, for example, keeps icons for the Bookmarks menu entries. Finder keeps the icons for the specific locations under the Go menu.
iTerm2 doesn’t seem to be observing this setting as of v3.6.9, but a fix is in the works.

As Daring Fireball observes, Safari needs 26.4 to observe this preference.