It was one of the tools that allowed me to maintain healthy credit card usage habits. In my idle time when messing around on my phone I’d gotten into the habit of just transferring money from my checking account to my credit card, often in excess of my actual balance, so as to cover “pending” purchases.
Not only will my bank no longer let me do that, just to twist the knife a bit, they will let me make multiple payments totaling the pending total due so long as no individual payment exceeds my actual balance.
So it’s not that they can no longer handle negative balances on the card, it’s just that they’ve half-ass made the interface disallow it.
FYI - your Credit Union and your Credit Union’s Credit Card are likely two separate and unaffiliated companies. Most small to mid sized banks and credit unions don’t actually issue credit cards. They contact that out to the credit card arm (ex. FIA Card Services) of a national bank (ex. Bank of America) that offers white-label/rebranded cards.