I generally agree with this sentiment ... but while a likely good rule-of-thumb, you also have to know when it's NOT good to apply
If you have version control, it's almost always correct - delete away, and revert if you need to
But when you don't ... commenting-out (and dating when it was commented) ends up serving as a [very] poor man's version control
I end up having to do this (or make copies and only work on the copy) with the product I use most for work because it has NO concept of version control anywhere ... and the add-ons that attempt it do it very poorly
...or you're overconfident, and understand it far less than you think :)
(not that I ever did something like that when I used to program...)