So I'm right to blame Slack for overloading an already existing shortcut??!? I've always blamed Slack, but I didn't know it actually was Slack that started this madness.

Too humble to mention that you're the creator :) thank you Ben! In my opinion, Slack is the application that really popularized the command bar/palette in the mainstream.

Popularized, probably true. But now I wonder which (maybe default?) keybinds I had for this in tiling window managers before Slack existed. And no clue when and how OS X introduced cmd-space.