You and DeCapua are correct, though some books still seem to believe in an absolute rule of backshifting that was never absolute.
As to your question: What about future tense? Would it be optional if something will still happen in the future?
Well, if the situation is in the future, nobody but a god can know that it will happen for sure.
However, expressions indicating futurity, which are in themselves present, behave as others do:
"I will come/am coming/am going to come on Friday."
1. He said that he will come/is coming/ is going to come on Friday.
2. He said that he would come/was coming/was going to come on Friday.
#1 is acceptable only if uttered before the Friday referred to.
#2 is acceptable regardless of when it is uttered, though if it were uttered after the Friday referred to, some clarification might be added, e.g. the following Friday.
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