What is this in that case?

They rejects them because it is an abuse prevention mechanism. You can solve captcha and register without any additional information

Your can simply block sender, not only delete aliases. So it is a problem of webmaster that doesn’t respect users wish to unsubscribe

Yes. That’s why I asked help :(

You can try by yourself (without “grey” ip)

Unfortunately, I contacted support of some local websites, and they said that they wouldn’t white list any domains and I need to contact this list owner

Yes. Aliases is not disposable mails. Moreover SimpleLogin (Proton aliasing service) forbids to use it as disposable in ToS

Also this list blocks it too https://github.com/wesbos/burner-email-providers/issues/422

Please help!