While the new Mac mini with the M1 chip is only available with Gigabit Ethernet, Apple has listed multiple M1-based Mac mini logic boards with 10 Gigabit Ethernet in an internal parts list for Apple Authorized Service Providers.
For every Mac mini logic board with Gigabit Ethernet in the parts list, obtained by MacRumors, there is a corresponding logic board with 10 Gigabit Ethernet:
While the boards are available to order (in limited quantities), it is unclear why at this point. There are several possibilities, including that Apple listed the parts mistakenly or before it decided against offering a 10 Gigabit Ethernet option. The logic boards could also be designated for special circumstances, such as enterprise use. Apple could also choose to release an M1-based Mac mini with 10 Gigabit Ethernet at a later time.
Apple continues to sell Intel-based Mac mini models with optional 10 Gigabit Ethernet for now.