'Cause I did not receive any email about this so I do now exactly what happened. Anyway, if you want to add a Pinterest app to your Shopify store, here is a solution for you.
Hope it helps
I never got an email - only a notification on my store admin that the current Pinterest app was going to be disabled on March 30. Like you, I initially got a "page not found." Tried again via another route and got this page: https://apps.shopify.com/pinterest-1. It told me that I "needed to allow cookies in my browser in order to access the app from within Shopify." I added the app's cookies, www.s.pintergration.com, and www.pinterest.com to my browser permissions. Nothing happened; it just went back to the app page. After a while, I tried again and was able to add the app. It immediately auto-connected to my business Pinterest page. So I guess I'm good as far as my buyable pins are concerned.
Both Shopify and Pinterest could stand to be more helpful and transparent about WHY this change is being made and exactly WHAT the change entails, and Shopify could stand to ensure that all links work the first time and all requirements for logging in are spelled out clearly up front.
@CuriousMoose, what kind of device are you using? What operating system? What browser are you using? Do you have any security or privacy add-ons that might be blocking cookies or trackers? (I've had to whitelist all the social networks that I use for sales channels, but since the Pinterest app uses a slightly different domain from Pinterest itself, I had to manually add the cookies to my browser and cookie-blocker whitelists.) Are you getting any error messages, and if so, what are they? Do you have a business Pinterest account? Have you connected it to your store?
No clue, then -- I was just trying to think of possible trouble points. Was your store already connected to your Pinterest account or is this something you're doing for the first time? (Mine's been connected for a few years and I don't even remember what I had to do to get it set up.) You might try filing a help ticket with Pinterest just in case the problem is on their end -- though my experience with their help department is that you have to keep pestering them for a while before you get a human being.