Ctrl-[ is escape in ASCII (not specifically ANSI/VT100/etc), and Ctrl is sometimes abbreviated as ^.
I don’t know the exact history of why this is a thing that comes up with vi often. My guess is that ESC on the ADM-3a terminal that vi originally targeted influenced it. On ADM-3a, ESC is where tab is on a PC-101 keyboard, a comfy key to hit while touch typing. When later terminals started to move ESC a couple of rows up it was more ergonomic to hit ctrl-[ instead.
Ctrl-[ is escape in ASCII (not specifically ANSI/VT100/etc), and Ctrl is sometimes abbreviated as ^.
I don’t know the exact history of why this is a thing that comes up with vi often. My guess is that ESC on the ADM-3a terminal that vi originally targeted influenced it. On ADM-3a, ESC is where tab is on a PC-101 keyboard, a comfy key to hit while touch typing. When later terminals started to move ESC a couple of rows up it was more ergonomic to hit ctrl-[ instead.