Yes, Senator John McCain is still relevant. I don't always agree with his stance, but in this case I do. If another country interfered with our elections, that is extremely serious and problematic. McCain is correct in wanting to investigate possible interference by Russia. He is therefore still very relevant as he raises important issues and doesn't just sit quietly.
Nobody should say McCain is irrelevant. As far as am concerned, he is very a very influential and has a great following across America. So, whatever he says should be taken seriously, even if it is a sensitive matter like this one where he doubts the credibility of just concluded election and and calls for investigation to establish the role of Russia on the outcome.
A sitting US Senator, especially one that has been in office for so long, is not only relevant but we should listen to his advice. He has every right to ask for an investigation into Russian interference, as a long-standing member of one of the three branches of government that are here to serve.
McCain had his chance. He ran for President. He didn't even come close to winning. He wasn't a viable candidate because he was just another old, white male that wanted to impose patriarchy and Rhino Republicanism on the rest of the American people. John McCain slunk back to the Senate where he insults people that do a better job than he ever did.