This may be one of the most overused and misused quotes in martial arts. It is usually attributed to Bruce Lee. One has to decide what “useful” means to them. For one person, it may be self defense, for another it may be sport fighting, for another it may be fitness. etc etc.
That being said, more specifically, most want to discuss this in a fighting context. Sadly, many use the quote as an excuse for not attempting or acknowledging the possible “useful”-ness of a technique outside of their own choice moves. It is possible (and probable) that Bruce Lee gave most of his eventually-discarded techniques a thoughtful test-drive before he chose to not “absorb” them. And it is because he dedicated some time to these edits, that he developed solid thoughts on how to counter them and/or decided that he already had something in his arsenal to render them redundant.
It doesn’t mean that exact discarded technique couldn’t be completely “useful” in someone else’s personal martial arts mix.
It’s like all the thoughts, drafts, edits, and revisions an author wrestles while writing a book….or martial arts philosophy.