On Wikipedia Rancid, NOFX, Penny Wise all have a similar paragraph about all the other bands that came out of the same area and Green Day is in all of them.

Green Day may have shifted over time into Rock or Pop Punk as you say but make no mistake when they originated they were Punk. Especially if NOFX, Penny Wise, and Rancid is. The problem is that when a band hits it, they are usually tagged with whatever they were defined as from then on. So they are punk because they were originally punk. I know I am arguing and I am not meaning to. Just trying to point out that there isn’t much reason to say they aren’t punk anymore.

It’s like my argument about “hover boards” I lost that one a long time ago and it doesn’t help anyone to keep correcting people an say it’s not a hoverboard. The name stuck from the start.