What Are the Potential Censorship Concerns Associated with Private Transaction Relays like Flashbots?

Private transaction relays introduce censorship concerns because a block builder or validator could choose to exclude specific transactions (e.g. those related to sanctioned entities) from the blocks they build, even if the transactions are valid. Since the transaction flow is hidden from the public, this selective inclusion/exclusion becomes less transparent and harder to monitor, potentially compromising the network's neutrality.