Yeah, twitter requires that to check for authenticity. There's also a lot of abandoned twitter accounts out there with 100k+ followers which have been registered with a custom domain name email addresses that also expired. You can hack them by paying and registering the old domain name and recreate their email address from there again. One of the basic blackhat practices out there, you can also see some brands that do this to steal a username from an abandoned account, if they have custom domain email.
On Instagram, you can do the same thing. However, things got differ from banned accounts. A lot of marketers experience several bans, and they received countless appeal, and to make it hard for marketers to recover they basically make their reporting system crap. It suppose to be the original email, it is also indicated there that you should put the email that is associated with the account, but I just discovered that it was programmed not to response to your request at all, and by putting a different email address there that isn't associated with instagram yet, they see it as a priority request, like inviting a friend or a contact via email that isn't on instagram yet, by doing that you will receive a response. You can experiment too, try to submit two request, put the original email there, and put a different email on the other, you will receive a response on the email that isn't associated to the account, pretty stupid system, but this is what i've discovered that works.
