How to get SMC version for 2018 Mac mini
On Macs with the Apple T2 Security Chip, the System Management Controller (SMC) is part of the SoC of the T2 chip and does not report the SMC version like the Macs without the T2 chip, which is why you do not see it under Hardware Overview.
You can select Hardware > Controller to see the firmware version of the Apple T2 Security Chip.