With id editor, if you switch on the Locator overlay (right panel “Background”, part “Overlays”), you can see here: OpenStreetMap a dotted line which split the island in a north part and a south part.
If you zoom in: OpenStreetMap, there is no line.
Where is this line ? Is it possible to have/see it on id editor ?
The Locator Overlay is a custom Mapbox map that overlays the Mapbox Streets source tileset over a transparent background. What you’re seeing is probably the boundary between the Baie Sainte Anne and Grand’Anse Praslin districts, which hasn’t been mapped in OSM yet. The Mapbox Streets source gets its boundaries from non-OSM sources. The overlay is only intended to give a cursory idea of what you’re looking at, so you can quickly navigate to a place of interest. You shouldn’t trace boundaries from this overlay into OSM, but this sounds like a boundary that might be worth mapping from another source.