I have used P.S., on and off, most of my life. I have never done so because I forgot something and had to add it after the fact. When I've used it, I've done so deliberately—specifically wanting the text to go at the end (and after my signature) rather than anywhere else.
So, for me, it's always been a matter of stylistic choice.
Karen Hertzberg expresses the same idea in the blog post "What PS Means and How to Use It Correctly in Your Email". (Note that the UK form, and that used by The Chicago Manual of Style, does not use periods between the capital letters.)
PS once saved us from having to edit or rewrite an entire letter just to include an important afterthought. But email allows us to go back and edit before sending. Technically, we could avoid the use of PS altogether in electronic communication. But should we?
Not really. PS is still useful for effect, and it’s a great way to get a specific point noticed. Although the Internet has made us a culture of skimmers rather than people who read things like email word-for-word, we tend to notice what’s at the beginning and end of a text. Can you think of a time when you didn’t read the PS in an email you cared enough about to open?
Including a PS has long been a direct mail marketing strategy. Statistics once showed that as many as 79 percent of people who opened a direct mail letter would read the PS first. Although times have changed, email marketers still swear by it as a way to reiterate a call to action, create FOMO, provide some sort of bonus information or offer, or even share a testimonial.
I thought about this, and those statistics reflect my own reading habits. If I see a message that has a P.S. at the end, I tend to read that first—and then read the message itself afterward.