Trumpet. I spent a year straining for high notes and gained almost nothing. Switched to patient lip slurs and air work and the range came almost as a side effect. What genuinely extended your range - and what was a waste of time?
Not brass, but the parallel holds for sax altissimo: it’s air speed and a relaxed setup, not force. The notes you strain for are the ones that never stabilise.
Flute top octave too - it arrived when I stopped blowing harder and started focusing the air faster with support. Straining just builds a ceiling.