How is Ctrl + / working as undo in Bash 5.0.3 on Debian 10.0?

Question

I have a Debian 10.0 system. It has GNU bash, version 5.0.3(1)-release (x86_64-pc-linux-gnu). In Bash prompt, I type

foo bar baz

Then I type alt + backspace two times. The words baz and bar get deleted and only foo remains.

Then I type ctrl + / two times to undo the deletions. The words baz and bar reappear.

How is ctrl + / working as the undo command? I don't see it defined anywhere?

$ echo / | grep -E "/|undo"  # just testing my regex
/
$ bind -p | grep -E '/|undo'
"\e/": complete-filename
"\C-x/": possible-filename-completions
"/": self-insert
"\C-x\C-u": undo
"\C-_": undo