Most transactions yes. But most of the BIG transactions, no.
Okay, tax evasion/avoidance is always done through other means. But other illegal activities, yes it is predominantly used (or was, In not sure about right now). Now nothing online is untraceable obviously, but it’s much harder to trace than say through a credit card where your ID is linked to the number. The only surefire way to be untraceable is through tangible cash, which you can’t pull out more than a certain amount at a time (in the US) without getting the FBI involved. And even if you don’t pull out enough for them to notice, they can still catch you if they really wanted to via other means. Cryptocurrencies are the safest bet, if you are extremely tech savvy.