Can a 2018 MacBook Pro run macOS High Sierra 10.13 natively?
2018 MacBook Pro originally shipped with macOS High Sierra 10.13.6.

It is possible to downgrade it to run the said release of macOS High Sierra on your MacBook Pro.

You can install macOS High Sierra 10.13.6 by downloading it from the Mac App Store, creating, booting and installing it via a USB installer. Alternatively, you can also use Internet recovery to directly download and install it natively.