Usually that ends up with Devs saying "if you have issues on these platforms we will no longer support you in fixing them and it will get no more updates from us". Sadly this is then going out of their way to stop people with old OSs from playing their games, even though they would still technically run.
Now if people want to use an old Os, for whatever reason, they will now have to pirate those games even if they do own them. And that itself is what would put those users to risk when using an old OS is they don't know where and what to download safely.