Learn English – Is this sentence correct : “I would have never believed that it would become like that”

Inversion for conditionals is used for a more formal style. When omitting the if clause, you apply inversion in the beginning of the sentence.

Had you not told me that, I would never have thought so.

These distinctions are simply guidelines but not hard-and-fast rules. The shift of tense does not indicate that the event has not taken place.

Let's say that she agreed last week to have lunch with Susan today, to discuss a business proposition. The lunch does take place. She has a sandwich. Susan has soup. If the question is about what she said, it doesn't matter whether the lunch took place or not:

When I spoke with her on the phone, she said she would have lunch with Susan today, and they did indeed have lunch. She's a woman of her word.

Not needing to change the tense when the assertion is still true does not imply the corollary that changing the tense means the assertion is now false:

He said his brother worked in Milan.

His brother may still work in Milan. The shift of tense signals that the speaker is reporting the speech of another, not making the assertion himself.