I was born in the 60’s.
I played little league ball, watched hundreds of games as a kid and an adult, went to dozens of NASCAR races, and currently coach little league ball.
I have never once heard a team or the spectators sing along with the National Anthem at any venue, game, event, concert, or party.
Not.
Once.
It is a pretty much limited to soccer. The reason is during the playing of the anthems the cameras are focused on the players faces, and the mikes pick up their singing, so I believe it’s one of those things that after one team did it, all the others fell in line, and it became the expected thing to do.