A well known and tested approach, sum types (tagged unions), would have been way better, but it was cast aside because the designers were apparently unaware of the improvements to union types made since C's inception:
There is no reason for any new language to lack tagged union types. It disturbs me that Brian Cox rejects a simple, proven language feature that he does not even understand, even though it would take all of 2 minutes of searching/reading to understand. I'm sure he spent at least that long composing the replies in that thread, never moving past the ego-threat of "will Go ever have X?" to honestly evaluate the question.
"There is no reason for any new language to lack X" is false for all X. Languages differ in their goals, and there's no feature that all languages have to have. Even basic features like assignment can be questioned.
You're jumping to a false conclusion that Go's designers were not aware of those language features, and that that was the reason why they aren't in Go.
You said that sum types were omitted from Go because the designers were not aware of more recent developments. That's not true. They were omitted because they do not mesh well with the other features of the language, such as zero types, interfaces and embedding.
Whether you agree with that latter point is moot. Go's designers were and are fully aware of sum types; they chose to omit them from Go for a reason, not because they were ignorant of their existence.
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