I just came across an article in the help/info pages entitiled "Account Suspension - Skills Are Not in Demand." It contains this passage:
"Are you closing client accounts too? Some clients post many jobs but never hire.
We have systems in place to check both client and freelancer accounts to make sure they are not violating our Terms of Service and suspend both client and freelancer accounts to improve the success of the overall Upwork marketplace."
Does anyone know where in the Terms of Service the issue of clients listing jobs but never hiring is addressed?