let go

I’m not sure what they meant by ‘let go’ in the excerpt: “There’s a two-footed tackle in the second half let go. I’m amazed by the referee but that’s part and parcel of the game." (It was not penalized of course.)

Here’s the key to your answer:
“It was not penalized”

The referee did not take any action regarding the illegal tackle.

Similar to ‘let it go’ meaning ‘don’t concern yourself with it any further, just move on.’