It’s not just support for Server 2003 that finished a few months back. Vendors are unlikely to support applications running on OS unsupported by the vendor.
Most versions of applications designed to run on Server 2003 won’t run on Server 2008 R2 or Server 2012 R2. To support these new OS, vendors have released subsequent versions of applications. For example, version 6.0 of an application may have run on Server 2003, but wouldn’t run on Server 2008, so version 7.0 was released for organizations that were using Server 2008. Unfortunately in many cases version 7.0 would not work on Server 2003. The vendor kept version 6.0 supported for the customers that chose not to upgrade, supported version 7.0 for the customers that were running later OS and so on.
While it makes sense for vendors to provide support for all those version 6.0 applications out there while it still pays the bills, at a certain point in time vendors, like Microsoft, eventually flip the switch and suggest that clients upgrade.
Of course most vendors that had applications that ran on Server 2003 pulled support for those applications quite some time ago. Organizations still running the applications have instead taken a “hope it doesn’t break” approach to running the workload. They know that they don’t have an escape route if something goes wrong, so they just keep hoping that nothing will go wrong until they somehow find a way to retire the workload.