They surely wouldn't have approved my decision.
This sounds like perfect English to me.
But after searching on Google, I realized I was wrong, that I had to write instead:
They surely wouldn't have approved of my decision.
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Both sentences exist but with different meanings. "They surely wouldn't have approved my decision" means "they would not have agreed to accept it" so they had the authority to reject it. "They surely wouldn't have approved of my decision" means "They would not have considered it to have been a good decision". So they didn't like it but couldn't necessarily have it changed.