I’m just estimating with the 1 in 5, and it is only my personal experience. Also, that’s not really how probability works. Sometimes you might get 3 toxic players in one game and others, none. In my experience, I go 2-3 games with everyone being friendly, and then you get a game of people arguing. I’d say 1 in 10 games, you get someone truly abusive. Despite the player-base’s short term memory, I think toxicity is not as bad as it was in the beginning. Reporting/actioning and endorsements have helped (but they will never completely prevent toxicity).
I’ve been going Brig against Sombra if I’m support and can’t defend well enough as Ana. It has worked out for me most times even though I’m not a great Brig.