[Twisol]

Yes -- sum types have historically not been present in imperative languages, whereas products types have essentially always been present. Imperative languages have correspondingly good support for various uses of product types, but even if sum types can be encoded, you generally can't abstract well over them due to the nonexistent language support for their use.

C does have unions, but you can't distinguish which field is safe to read without additional information. Discriminated unions are a design pattern built on top of the language feature to give a true sum type, but since they aren't core to the language, pattern matching still isn't a thing.

(Technically, "pattern matching" encompasses both dispatching on sums and destructuring assignment on products, and to be fair, not a lot of imperative languages have good support for the latter either. But many do!)

[eeereerews]

Historically imperative languages did have tagged unions (Algol, Pascal, Ada, Modula). Their disappearance is mostly because of OOP's takeover I think. It was thought that a tagged union should be replaced by a base class with one derived class per variant.

What they lacked was usually a nice way to operate on tagged unions, ie. pattern matching.

[Twisol]

Thanks! Always interested in learning more about the historical PL landscape ^_^