The problem is that this is a critical security feature, so the consequences for users are rather severe if you rely on it but their browser doesn't support it.

The problem is that this is a critical security feature, so the consequences for users are rather severe if you rely on it but their browser doesn't support it. Instead of your site simply being broken or a certain feature on it not working, you'd instead be opening up all users on older browsers to a CSRF attack against your site.