Has Unix Parameter Name SystemAudioVolume Been Changed to SystemAudioVolume DB in El Capitan?
Firstly, the sudo nvram SystemAudioVolume=%00 (or the same command with different values at end) never allowed users to actually change the startup chime sound. In earlier versions of Mac OS X it did allow users to change the volume, and therefore they could mute the startup chime, but this did not change the sound.

The closest thing to changing the sound is a hack that allowed users to mute the startup chime and then add another sound that occurred upon logging in to your user account - but of course this is not a startup sound, just a sound that plays when you log into your user account.

As for whether SystemAudioVolume has been replaced with SystemAudioVolumeDB, the answer is no. In fact, as you have seen from running sudo nvram -p, these two parameters have totally different values.

Summary


No, the Unix parameter SystemAudioVolume has not been replaced by SystemAudioVolume DB in El Capitan
The Unix parameter SystemAudioVolume has never been used to change the startup chime sound - only to change the volume level
The startup chime sound cannot be changed


Further reading

These related questions may interest you:


Disable OS X startup sound
How can I silence the startup chime on El Capitan?