Some reputable casinos use third party verifiers. Your documents will be sent to those verifiers and will remain encrypted on their servers. Those verifiers are licensed companies and according to the laws they will delete all your data if you request so.
What's funny is that a random casino asked a customer to provide his documents through live chat! How reliable is this? Such casinos should never be trusted.
Some casinos that are new don't do things in a proper way, what you said is an example of that. The support team asking customers to provide KYC documents through live chat is obviously one of the least expected things in a casino or any kind of platform. It shows how nonauthentic and untrustworthy a platform actually is and they can definitely not keep your data and information safe if you complete your KYC verification with them.
Third-party verifiers might be trusted sometimes, but some of them might not have the reputation required for people to have complete trust in them. That's why it feels much better if a platform is doing the verification and stuff themselves instead of using a third-party verifier.
Well, I think I would have to differ a bit from your opinion, personally, as a user, it is hard to tell whether the casino themselves have the facility to keep your data safe, and it is also hard to believe same for the third party verifiers, both can be vulnerable to hack and both can be safe, but then, Ive always believed that one of the reasons why casino employ the services of a third party to handle their customer verification is because they themselves might be too busy wit other stuff, and might not have the facility required to properly keep customers data safe.
And knowing that the third party's main focus is on verifying users and making sure that the data is kept safe, it's believed that they are more likely to protect users data than the casino themselves.
Personally, I think that if the casino had the required facility to effectively carry out users verification and keep the data safe, they wouldn't have gone outside to pay third party verifiers for the service.