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.
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.