The only proof I can find that the rules against the Hittites were still in
effect in David's time are from King Solomon's time. 1 Kings 9:20-22 and 1
Kings 11:1-2 say that they were supposed to destroy the Hittites and not
marry them, that is why Solomon made them slaves and also why he got into
trouble for marrying some of them.
These rules were not mentioned in the story about King David's sin, but that
does not mean they did not exist then.