Mac Mini (2018) 10Gb Port Compatibility
Yes, it is compatible with both 1000Base-T and 100Base-T. I own the Mac Mini 2018 with 10 Gbps option and have tried both configurations in practice.

It doesn’t appear to be compatible with 10Base-T, but you’ll be hard pressed to find such equipment anywhere these days.

You can find Apple’s description of the network support at the following link:

https://support.apple.com/en-us/HT208405

It similarly describes that 100Base-T is a supported configuration, while 10Base-T is not. Look at the included table for instructions on which cable types to use for the various supported speeds.