“Needful” means necessary or required. Some antonyms for “needful” are “unnecessary”, “superfluous”, and “dispensable”.
“Unnecessary” implies that something is not needed or is more than what is required. For example, “Buying that expensive gadget was unnecessary when you already have a similar one.” This shows the contrast between what is needful and what is not.
“Superfluous” suggests being more than is needed or excessive. “The extra decorations were superfluous and only cluttered the room.” Here, “superfluous” emphasizes that something is not just unneeded but also perhaps even a burden.
“Dispensable” means capable of being dispensed with or not essential. “Some of the old rules seem dispensable now as times have changed.” This indicates that something that is dispensable is not needful and can be done without.