Bridge Chestnut 6 - "A change of suit by an unpassed responder is forcing"
This chestnut is by partnership agreement. It is meant to apply to uncontested auctions not starting with 1NT or a pre-empt. When our auctions start with 1NT or a pre-empt, our response have different meanings.
Here is a typical auction where opener must keep bidding. Responder’s hand is unlimited.
On the contrary:-
This 2D bid "begs to be dropped". Responder has shown less than 10 HCP, at least 6 diamonds and hopeless holdings in opener's black suits.
Whether 2D is forcing depends on partnership agreement. Here, the opponents have overcalled at the two level showing a goodish hand. Many pairs play 2 here as a Negative Free Bid showing about 8 – 11 HCP and a 5 card suit, not forcing.