On the other hand, I just learned from your link that there apparently were A500s where the low pass filter could not be switched off in software. AFAIR, toggling the LPF in software was already possible in the predecessor, the A1000.

Anyhow, coupling the LPF to the power LED (and having a software controlled power LED in the first place) is a very Amiga thing to do. It's one of the platform's many nice idiosyncrasies.