Backspace with any modifier always is send as back space.

$ showkey -a
^? 127 0177 0x7f -> alone
^? 127 0177 0x7f -> with ctrl
^? 127 0177 0x7f -> with Alt
^? 127 0177 0x7f -> with ctrl + alt

I have it bound to delete word in bash and only can make it work (one at a time) with Esc + backspace.
Esc + backspace, esc + backspace, esc +backspace,... works.
esc+backspace + backspace+backspace doesnt' work either.