DiscardPadding support was added for Opus, but it can apply to Vorbis too. See bug 1069660 comment 8.
Do we need this in the WebMDemuxer? if not I will close this as won't fix.
How does it affect users?
Ralph can probably explain this better than I can.
Basically, without this you can't cut from one encoded track to another without a gap in the audio in MSE. WebAudio resources will also need manual trimming. This element is needed to support sample-accurate durations. Without it webm only represents vorbis audio up to the nearest frame boundary, and unless the edit is aligned there or a special crossfade cluster in encoded, there will be a gap or click at the transition. Our implementation will be lower quality and frustrate developers. The Ogg container has always supported this, but the field (along with CodecDelay and SeekPreRoll) was added to Matroska for Opus in WebM.