Install Sierra on Macbook Pro shipped with High Sierra
The well-accepted answer now and since there have been Macs (I believe...) is that whatever OS ships with it is the earliest version of the OS that will go on it.

So if it ships with macOS 10.6.8 then there is no way you will ever get anything lower than 10.6.8 on it (not 10.6.7, not anything).

So to answer your question specifically, if it came with High Sierra (whatever version) that is the lowest version of macOS it will ever run.

There may be hacks around that (and that answer won't be here), but I am not aware of any and I have been supporting Macs since 1990, professionally since 1992.