Well, Pascal has been dominant for a short time before C and its family really took off. But that was such a long time ago that I kind of forgot about it. C (without ++) was nice because you could write code without too much boilerplate compare to Pascal and it was noticeably faster (computers were slow.) However any mistake with pointers could crash the computer if it was a DOS PC and not a UNIX workstation.

C++ has std::variant, but that's pretty new.

My understanding is that's not really a sum type, though -- e.g., you couldn't use it to add a type to itself, only to add distinct types to each other.

No, you can add a type to itself (that is a pain in the ass though). You might be thinking of the fact that it may not hold a value (eg. if an exception is thrown while moving into it), but they made it as close to a sum as they could given the nature of C++.