This, but for many things.
Paint is ready at the hardware store Table is ready at the restaurant Construction is done on a bridge
All kinds of things that we need a one-time notification for.
That was the first thought that struck me when I saw the headline and tried to guess what the posting was about before clicking the link. At least 2-3 times per week I find myself wanting something like this. Often it involves leaving my email address or my phone number to have someone contact me. Or having to check back.
In its simplest form, this isn't all that hard to build. The tricky bit is to get people to use it. And perhaps even to explain what it does and possibly how it works.
If someone knows how to sell it, I'd be willing to build it.
It was conceived for social networking, but social networking doesn't have to be just 140 characters in a timeline.
https://www.w3.org/TR/activitypub/
Your suggestion is a valid one, but I think the best way to solve something like this is to _first_ look at how you'd solve it from scratch. Or at least try to figure out what problem you actually want to solve.
I usually avoid looking at how other people have solved a problem until I have had a chance to think about it myself. Then, when I have given it some thought, I go back and look at what other people have done. This occasionally leads to novel ideas.