What's meant by "what has come to"
Mrs Thatcher firmly believed in self-reliance and what has come to be known as privatization.

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In this sentence "what has come to be" means "what has become". Saying that something has "come to be" means that it has acquired a trait over time, in this context implying that Mrs. Thatcher believed in the ideas behind privatization before that set of ideas had the name "privatization".

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what has come to be known as privatization. >as something that we now called privatization

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It means that it is known now as 'privatization' , before it was not known as this.