High resolution files on old DACs?

[gvl]

It has an 80 SINAD, I wouldn't worry about any of that

Yet they dare to say that, this is what happens when your marketing guy is also your technical writer.

 

[Vincent Kars]

What happens when I feed an older DAC (16 bit / 44,1 kHz) a higher signal through USB?

Depends.
Part of USB is the device enumeration. Your DAC will tell your PC is is limited to 16 bit/48.
If you go into the Win audio settings this or 44.1 is all you can choose.
As long as you use Direct Sound (or WASAPI/Shared) Win knows this and resample every thing to the sample rate you have set.

If you use WASAPI/Exclusive, you bypass the Win audio stack completely. Old (say 2015) implementation of WASAPI/Exclusive will simply crash the moment you send a sample rate / bit depth /number of channels not supported by the audio device.
Modern implementations might do the same as Win, check the device enumeration and warn you if you send something not supported or resample under the hood.

Bit more about USB audio: https://www.thewelltemperedcomputer.com/KB/USB.html

 

[gvl]

Players that support exclusive mode can typically downsample before sending data to the device.