In your case, most likely, past transactions were taken into account, which turned out to be more than $ 1000 in total (according to the new rules), and now any operations with your account have been frozen. From this we can conclude that if someone wants to use OKX now in the most secure way, he should create a new account 9while there is such a possibility), or use old accounts that have not had any operations before (as in my case above).
I agree, I don't know why they didn't mention that, meaning they should have said, the deposit limit is $1000, you have $400 left.
What country you are living? It seems they don't offer service in the US due to local law according to OKX support post below
UK. But it seems that what happened is not local, but for all accounts.