Depending on how the mixer is presented to the operating system, a third-party program may be required, yes.
However, if the mixer is presented as an audio INPUT device, then OBS should be able to capture it just fine.
Nothing has changed with the audio device support in macOS or OBS for quite some time, not sure why you're using a term like "crippleware" here. Any device that presents as a desktop audio device cannot be captured by OBS, and never was able to be. There is nothing we can do about this, or we already would have. Apple has not provided, and shown no interest in providing, desktop audio capture methods. This is why a third-party program is required, and due to this being a global issue on macOS and not an OBS specific problem, it doesn't make sense for us to reinvent the wheel when there are dozens of solutions to this problem already available (both free and paid).
If the device was working properly before, it should still work. I would check any configuration utilities the external mixer uses, and verify that the device has not changed output modes.