Do you think it's normal for a sportsbook to investigate for more than 6 months?
Well, investigating for more than 6 months does seem like quite a long time. But I have seen such cases before. It probably depends on the type of investigation and who is conducting it. It may be a third party security provider so it is out of the casino's control. It does sound a bit on the longer side, though. A month or two should be the maximum in my opinion.
As far as I know, they used to have an active campaign signature (where I participated for more than four (4) weeks) in this forum and the pay was relatively higher compared to the standard signature campaigns that are active currently. It is really weird that they provided a timeframe of more than six (6) months, which is somehow unusual.
I just checked its website and saw that there are active people who currently bet. You can see that the chat box has active people who frequently message, though I am not sure on how active they are.
If I were in your case OP, I would definitely reach out to sujonali (previous campaign manager of this website) or respond to their message and asking for questions on why it is that long before they can accept your withdrawal.