Is that sentence below correct? I have heard about the phrase "have a second thought". Do this sentence need an article "a" before "second thought"?
But in Bollywood, diversity is always something that's maybe been second thought.
No, it isn't. You can say either 'have second thoughts' ('thoughts' here is always a plural noun) or 'on second thought/thoughts'. See: https://www.collinsdictionary....glish/second-thought