I’ve left a match maybe 3 times from day 1 d1 to present day. The issue stems from others reporting me mid game and Bungie taking that as an excuse to send out a warning message when I loaded up this morning. Thing is I know I don’t have the best but I also don’t have the worst internet. Upload is a steady 10-20mbps, am I in the wrong?

Upload isn't the number you want. You want what your download speed is. If your download speed is 10-20, then there is a good chance that while you're in a match, it's occasionally dropping lower than that, and it causes you to lag around for other players.

Download stays around 300mbps