When launched, the Apple M1 Mac mini did not offer a 10GigE port. I checked yesterday, and now it has that as an option. I think this quietly got added after the Apr 20 event.
The new iMac doesn’t seem to have a 10GigE option, however. But that might change once it releases. But since the Ethernet port is built into the power supply, maybe one can get a new power supply with a 10GigE port in it, at a later date. A third party one, for OWC maybe?
I’m just guessing, but I think the 24in iMac will be aimed at the consumer level, and the new larger iMac (rumors say 30in?) will probably have that higher speed as an option. I would be cautious about any high speed network cable that sends data and a lot of power along the same path- they may peak at 1Gb and push wifi as the future anyway (if only these had Wifi6E!). So an external dongle is probably the way to go for 10Gb on the iMac. But I’d also love to see a TB3 dock that is also a a riser for the unit, or that can fit in the space on top of the stand’s foot.