Although it depends on the provider, unstaking ETH will not be allowed until after the Shanghai hard fork. Nonetheless, a derivative token called stETH (staked ether) is freely tradable in the meantime. In addition, once withdrawals are enabled, the exit rates for validators will be staggered by the protocol to help prevent any market fluctuation or security risks. According to the Ethereum website, only six validators may exit per epoch (every 6.4 minutes, so 1,350 per day, or only ~43,200 ETH per day out of 10 million ETH staked). Furthermore, although ETH will still remain locked for a period of time post-Merge, validators will have immediate access to the fee rewards/MEV (miner extractable value) earned during block proposals.