backward-delete-char-untabify-method is a variable defined in `simple.el'.
Its value is

The method for untabifying when deleting backward.
Can be `untabify' -- turn a tab to many spaces, then delete one space;
`hungry' -- delete all whitespace, both tabs and spaces;
`all' -- delete all whitespace, including tabs, spaces and newlines;
nil -- just delete one character.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 20.3 of Emacs.