Nikon Z8 User Manual page 687

e2: Flash Shutter Speed
G button U A Custom Settings menu
Choose the slowest shutter speed available with a flash in mode P or A.
Regardless of the setting chosen for Custom Setting e2 [Flash shutter speed], shutter speeds can be
as slow as 30 s in modes S and M or at flash settings of slow sync, slow rear-curtain sync, or red-eye
reduction with slow sync.
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e2: Flash Shutter Speed