I compiled a webm of probably 90% of the times I had people leave my matches (often times they leave when there was still a good chance to revive and it wasn't during the final rings).
Started grabbing recordings from around season 3 and only missed a few. A lot less leavers than I expected.
In fact, I'd say maybe even a 3rd of the time I had missing teammates it was probably cause of their game crashing or some sort of bug (not things I included in my webm if I suspected that was the case).