Discord is not a requirement it just makes clearing content more efficient. No way to type out commands mid fight in anything complicated so discord is sorta the only means of communication besides maby a danger bongos macro. When learning a fight hearing callouts of mechanics coming will increase the speed you clear it by 3-4 fold easily.

In the end is it possible yes, is it optimal no. There are groups for almost everything that do not require it, alot that make it optional. Expect to see alot of mandatory discord groups for upper end content. If discord is a no go, have to look for the groups without it. And expect alot of them to break rather quickly if they are learning parties.