Using "Islands" tool from Segment Editor in 2D

Islands tool is currently available for 3D. I mean, “islands” are computed as connected components in 3D.

Is it possible to set “islands” as connected components in 2D?

You mean you would like to restrict the connected components to a slice by slice segmentation (i.e connected only in 2D not in 3D). if so, no I don’t think that’s possible nor wanted in 3D Slicer.