[danadam]

> When I use an FFT to view the spectrogram on YouTube music videos, it is very obvious that YouTube applies a lowpass filter at 16kHz on all videos (true since 2023 at least).

Maybe with a browser that doesn't support Opus and gets AAC instead (Safari?). With Firefox or Chromium on Linux I get up to 20 kHz, which by design is the upper limit in Opus codec.