[TuringTest]

I'd say the closest thing in modern systems would be the Stream metaphor (a.k.a. Observer design pattern) in functional reactive programming.

This is the construct used to program the spreadsheet's reactive behavior, after all. You could think of users creating spreadsheet formulas as declaring a pipeline at runtime between two datasets, the origin range and the target range.

What is missing is a way to create "spreadsheet functions" among different applications. Online computational notebooks a la Jupyter are almost there, but they still need to share the same runtime kernel to transfer data between different books.

A combination of notebooks and web hooks should do the trick, methinks, but it needs someone to assemble this brittle system into a robust product.