My guess is that it will depend on the individual DAC makers and their individual products.
Most likely they may write updated Apple Silicon drivers for recent products as well as upcoming products but may decide it's not worth it to write drivers for legacy products.
Apple announced Apple Silicon at WWDC in June and these DAC manufacturers could have applied for a Desktop Transition Kit to help them with their driver development. Today, they can just order an M1 Mac mini like you or I.
Considering that Apple is developing Logic, Mainstage, Garageband, etc. for M1/Big Sur and beyond, it would seem likely that external DACs aren't going away but the ball is in the DAC maker's court.
Since it's just a USB device/port, seems like the hardware side of the port would be pretty abstracted, i.e., if the OS handles a USB DAC, the hardware will. You know, in cases where there's no additional driver needed (which has been the case with a number of external DACs I've used/tested/currently own).
FWIW, I've taken my external DAC, and connected it to my Mini USB-A to MicroUSB (that's the DAC side, also has Coax, TOSLink), a simple, $5 USB-C to USB-A pigtail (then a standard A to micro cable), and I've also connected it via a USB-C hub (A port on the hub, C on the Mini). Also, that same USB-C hub, works on our iPadPro to drive the DAC as well (provides charging, concurrently), that's actually where I use it day-to-day (bonus, it functions as a Nintendo Switch "dock" )
I'll try it when we get our M1 MBA, which may be late December <sad_face>