I do realize people have been using tagged unions for a long time. Having some library helpers goes a long way in making them more usable and more expressive. Having them built into the language as a first class feature is even better yet, but std::variant is a nice middle ground.

Technically you can implement it manually, but the same thing could be said about all language features, even in C. We don't need actually need structs built into the language, we can just allocate blocks of data and handle the offsets by hand. We don't need functions, we can just push stuff to the stack and use the call opcode. The same goes for various loop constructs, goto can do everything "for" and "while" can do!

I don't think "we used to roll our own X back then" is a strong argument for something being bad or unneeded. Abstractions are all about allowing the computer to do work for us, and making things less error prone and more expressive. This is why we have programming languages to begin with and don't write everything in assembly!