a) With GNU/Linux OS & Ubuntu in general in text editors
b) Specifically for askubuntu.com as to imply that the text is deleted (edited later)
c) or as an html formatting in a broader sense.
I want to edit something but I would like the old text to be viewed in that form so that people can have an idea on was written before because there are comments on the original version. (I even don't know what that formatting is named. Edit after JorgeCastro's answer: Appearently its called strikethrough)