In 2-node cluster using failover with Dynamic Quorum, is Witness needed?
Dynamic quorum basically dynamically adjusts votes depending on available servers.

Each time when one of nodes goes down, dynamic quorum will remove the vote from that node.
In your scenario you have 2 nodes only and dynamic quorum will automatically remove the vote from your passive node, so the 1st node will have the majority of votes. In planned maintenance scenario when you are shutting down the first node quorum will transfer the vote from first to the second, and remove it from the first node.

However in scenario where first node just crashes quorum does not have time to transfer the vote and your second node wont get to vote, which basically will just shut down your cluster.

Therefore in scenario with 2 nodes only, it is recommended to have a witness.