YouTuber Davie504 is an excellent bassist who takes on challenges leveled at him via his comment section. One guy asked for a 15 string bass song, and that guy got his wish! Davie504 wrote a very funky track with the bass, though it only goes so low before things get floppy and weird… plus, I'm not even sure the lower four strings of that bass are even functional.
Davie504 also looks pretty disappointed at their non-functionality too and seems like he only hits them to show you that they're useless. Nevertheless, dude crushed it! Now we just need to wait for Jared Dines to get his 17 string to out-string Davie504.