I listened to a 20khz sound and I could hear it, it was loud and at the same time not very loud (I maxed the volume). Probably because I'm pretty young.
Yeah, the drop off of max audible pitch you can hear is roughly between 18 to 20khz for most people on average, but if you are younger, just genetically have more sensitive ears, and/or have not been exposed to too much damaging sounds, I think some people can hear up to 22khz, but that's pretty uncommon.
Anyway, it sounds like you are not the kind of person who's had their dying smoke detector screaming at them so long that they can't even hear it any more, hahaha.
Made me think of The Melvins album 'Prick'. The final track has the intro 'And now, for your listening pleasure, a few moments of pure digital silence.'