The DLC was one of those “free to keep if you download within 1 month of release” promotions, but you had to be paying attention. XCOM2 did something similar with the Tactical Legacy Pack DLC.
Without the DLC, you’re missing some story, but maybe not $15 worth of story. I’m sure it will be included in some ridiculous “95% off BL2 bundle” when BL3 launches. Yes, they could have kept it as a free addon, but they have to appease the shareholders somehow.