Like this:
Why does the delete code now have strike through and pink background?
It used to just do strike-through, but now it’s got the pink background.
How come?
You can also indicate strikethrough with /s like this .
The delete code has been pink on the theme I use (Straight Dope Light) since the first day of Discourse. Note that it looks different on other themes.
Back on vBulletin it was pure strikethough.
@wolfpup has the right idea but muffed some of his formatting.
You can do pure strikethough by doing ~~strikethrough~~ which gives strikethrough.
Or equivalently <s>strikethrough</s> which also gives strikethrough.
But I’ve not changed my theme. It’s changed for me within the past couple of weeks. Before, it was just a strike-through.
There was an update to Discourse a few days ago. It trashed our ability post youtubes with visible previews. Perhaps it’s connected to your experience as well.
I’ve used the same theme since day 1 and don’t have any detailed idea how other themes look now or may have looked a month ago. I’ve fiddled with them in the past just enough to settle on one I like.
But I’ll bet that recent update has something to do with the change you see.
Suggesting that I muffed my formatting is sheer calumny! 
Actually, I had been having a nap (a privilege of being retired) and was posting from my tablet, with its crappy virtual keyboard, and that was just a quick and lazy way of indicating that text preceded by “s” in angle brackets and followed by “/s” in angle brackets would do the desired strike-through. As always, quoting the post in a reply would reveal the example formatting.
I believe the pink shading was indeed introduced in an update, but it’s older than the recent one. It’s been like that for quite a few months (at least, on the Straightdope theme I’m using).
When it started, it was just pink on the default theme. I actually used it for color. Since then, a strikethrough has been added. Fortunately, we now have Mathjax enabled.
I would never have interpreted that sort of shorthand as meaning that. Silly me.
I unreservedly and unconditionally withdraw my criticism of fine post, both expressed and implied. 
When I quote wolfpup, the quoted material has “like this” surrounded by two tildes on each side, so I’m unclear about the reference to a single slash-s and how you’d use that. I was told to use angle-bracket-s then the text to have the strikethru then angle-bracket-slash-s.
Test One (slash-s at end) Hello world/s
Test Two (anglebracket as taught) Hello world
Test Three (double tildes) Hello world
ETA: oh never freaking mind… I must’ve opened this tab longer ago than I realized
Are you sure about that? Because I definitely did not type that, I used “<s>” and “</s>”. If you’re actually seeing tildes it must in some strange way be a function of your theme. I’m using the Straightdope Light theme and when I quote that post, I see “<s>” and “</s>”.
Pretty sure:
ETA: using Sam’s Simple Theme here.
Fascinating. I’m using the Straight Dope Light theme.
When I attempt to quote the same post #2 as @AHunter3 just did, I see it with no tildes, but with <s> and </s> surrounding the like this part.
Examining the page source shows that post #2 is sent to my browser with angle-bracket s tags, not tildes.