Backspace key does not work in sqlplus.
It's not a issue form Oracle. This is a terminal type issue.
stty -a and see if you have done anything special with the backspace key.
I hope it’s working now.
Alternate way you can use.
Type stty erase in the terminal
Press CTRL+v on your keyboard
Press the Backspace Key on the keyboard (when I did this it put ^H on the line so my final line looked like stty erase ^H)
Now restart SQLPLUS you are able to use the backspace key to delete mistyped characters.