While the new Mac mini with M1 chip is only available with Gigabit Ethernet, Apple has listed several M1-based Mac mini logic boards with 10 Gigabit Ethernet on an internal share list for Apple Authorized Service Providers.
For each Mac mini logbook with Gigabit Ethernet in the parts list obtained by MacRumors, there is a corresponding logbook with 10 Gigabit Ethernet:
While the boards can be ordered (in limited quantities), it is unclear why at this time. There are several options, including that Apple incorrectly listed the parts, or before it decided to offer a 10 Gigabit Ethernet option. The logic boards could also be designated for special circumstances, such as the use of companies. Apple could also choose to release an M1-based Mac mini with 10 Gigabit Ethernet at a later date.
Apple continues to sell Intel-based Mac mini models with optional 10 Gigabit Ethernet so far.