I researched hate crimes for my dissertation topic and me and a professor spoke on Twitter after he wondered if there would be this reaction when fans returned to stadiums. It was easy making the gesture in empty bowls, would they continue to do it if it was loudly opposed etc etc
I doubt it'll be just Millwall. Just look at the annual hoo-ha that follows James McLean around. I expect the media to amplify it (over reporting it, the booing was clear, loud and obvious) being those Millwall ruffians but it'll be heard at other grounds too. Thy'd love to isolate it as a Millwall problem to avoid damaging the brand but I expect there to be at least one big incident of it in the Premier League.