Discord is encoding and streaming in realtime on top of a bunch of noise gating filters.

Opus is the most efficient audio codec we've got. (For stereo anyway.)

At high bitrates there might be better tuned ones, but also at high bitrates it doesn't really matter what you use as long as it's not MP3.

I'm not sure if it is the codec at issue there. I have only used Discord voice chat a few times being interviewed for podcasts and the quality at times is astoundingly good. That said, those doing the interviews are using higher end audio gear that most people would so that would go a long way to improving that.

Generally when I am grabbing audio off Youtube via NewPipe, I generally use the 50kbs OPUS stream if it is just talking/lectures. I cannot tell the difference. That said I once did a FLAC to 128KBit OPUS test on myself with music convinced I would be able to tell the difference. I could not. I just don't have hearing that is sensitive to codec loss it seems.