Needed? No. Preferred? Some cases vary. You can likely clear savage content without it but it's going to take a lot of wipes and instructions are never the clearest in text format as everyone doesn't understand your personality type through text(no matter how good you are at conveying emotion through writing). Personally I prefer no discord whenever possible for pugs and junk but if I'm going with a static or friends... I don't mind using it. Especially helpful in palace of the dead +101 runs too. Really comes down to the preference of the player, their patience and tolerance of other people. We have a lot of complete anti-socialists who refuse to hear so much as a word and we also have people who can't seem to shut up.. It's a mixed bag to sort through but the beauty of discord is, muting people is a thing.

Aside from voice chat being a thing... just having a text chat that's a constant out of the game is also very reliable in reaching out to someone you wish to play with again. Considering you can't always reach someone in game or they unsub or something.