What app was this? What's the context here?

So I clicked on it to answer your question.
Since I can't copy and paste the text, it says --->
"AdBlock Pro" $9.99

"AdBlock Pro is the best ad blocker for major browsers like Safari, Chrome, Firefox, Opera. Using this tiny app will let you block all ads across browsers on your Mac. We ensure that your browsing experience is not interrupted with annoying ads on any browser.
What Type of Ads AdBlock Pro blocks (sic)
Ads on YouTube, Facebook and other social platforms
Animated
Pop-up and Pop-under ads
Webmail ads
Collapsing / Expanding ads
Interstitial page ads

Why Install AdBlock Pro?
Ads are blocked on network level which means once installed, you need not install other heavy ad blocking apps.
There's no need to install ad blocking browser extensions across multiple browsers.
Light in storage, works with all browsers."

When searching for AdBlock Pro, I see that it is available elsewhere FOR FREE.
.

i wouldn't trust that message if i were you. it doesn't sound like something apple would send

[edit] ...just seen that it really is on the app store for 9.99, so maybe it is legit.
I can't see many people paying that much when there are so many ad blockers for free

Just one of the many dozens of ad blockers trying to get a piece of the action before it's all over... most are free.

It would make sense if it were for iOS, because most users don't care to take the jailbreaking route. But this is for desktop?!

AdBlock Pro is not being provided nor pushed by Apple (as the OP thread title suggests.) While Adblock Pro is offered in the iTunes Mac App Store, it is also available at all the other app stores (Chrome, Firefox, Opera, Internet Explorer & Edge.) Adblock Pro is owned and sold by PCVARK Software Private Limited, not Apple.

We're all paying heavily for the adblockers, even the free ones. Our personal browsing history and the ads we're being served is collected and extremely valuable data... and the adblocker users keep installing these apps and freely giving these companies access.

I have NEVER had a "buy this thing now from us right here" notification from the Apple Store, EVER.
Or any other kind of marketing pitch. You must go to them, they don't bother you.
And I have been doing Mac since before System 9 (six, actually). I still use 9 daily, and of course my OS 10.12.5 machine.

This was a first for me, and I thought that it might be interesting to someone. It sure caught my attention.
In this regard, it felt like a "push" to me.
.

I agree. I have never received spam from the Apple store either (my point.)

Probably just a spam email. The sender name is easy to fake. Did you source the header to see where it really came from? Could have come from the software company or an affiliate hoping to get commissions.

"Source the header" is greek to me. I am not a software person. In many areas, I barely get by.

This was not email.
It was a pop-up text message, as quoted above. It popped up when I was IN the App Store, doing a search for WYSIWYG editor.
.

Ah, then it was just an ad.

First one for me, from Apple.

Install the ad blocker and it will be the last! :)

"Source the header" is greek to me.

n/a here of course, since we're not talking about email after all, but in Mail, while a letter is open:

View menu >> Message >> select “All headers”

"Source the header" is greek to me

More accurately... "Source the header" is geek to me.