Never have.
But I’ve known a woman who dropped off kittens she adopted in the woods, because they were scratching up her furniture
I’ve also known a man who attempted to kill his cat by burying it because he didn’t like the cat and the cat didn’t like him
So people are monsters, but there can be legit, good reasons for giving your pet away to a shelter (not for dead, not abandoned, to a shelter). Eg, a newly adopted animal is causing a lot of stress to a current animal in your house, even after a year. Being unable to support your pet financially, who needs medical treatment. New job situation that causes you to travel a ton and you have a needy cat that loves people. Etc etc
But yea don’t abandon or kill