No, it isn't.
When the Developer's Cut was first released everyone who had D2:ED or DKS on Steam was given a free copy of D2:DC (basically DKS with some minor updates and a 'Developer Mode' cheat window added). Other digital distributors followed suit, and there was an upgrade program for a couple years to upgrade everyone else, including retail releases, to a digital copy from the Larian Vault. However, after changes to EU tax laws in 2015, the storefront function of the Vault site was shut down, and since that included the ability to generate and redeem voucher codes for games, the upgrade program was ended, as well.
