Challenge accepted. If a user (esp. one whose cognition generally prefers visual media) uses a menu item frequently, they can remember its icon and that makes it easier to find in the future.
(Doesn't apply to me personally though because I'll instead remember the underlined letter and press it next time. My pet peeve in menus is not icons, but missing or clashing hotkeys.)
They changed it recently for attachment to look like a paperclip on a document which is much better. But before, I almost always clicked on one when I wanted the other (or hovered my mouse over it for longer than I'd care to admit).
If you're going to use many icons, then they need to be visually distinctive. That means ditching the flat designs, and embracing colour again.
It is the reason for removing colors and shadings from all icons.
I would argue that menu icons are more useful than dark mode in several situations.
It seems some monitors now assume dark mode - only the very lowest brightness setting isn't blinding.