couldn’t find it anywhere , google doesn’t have an answer either.
I think this icon is the one you’re looking for.
I still can’t find blender UV sync selection in blender 2.9. I am even in the UV editing mode and they still do not appear.
It is like @xrg stated…it is the double arrows in the top bar of the UV edit window…
You have to select the mesh and enter edit mode… then click on the arrows and you should see all the UV islands appear…then I personally use Face select as it is a bit easier to work with. In either select NONE…
Now you can hover over any part in the viewport or in the UV and hit L and it will sink between windows…
I think I know what you are talking about. I don’t know if it is a bug or not but Blender 3.0 has sometimes given me a UV window without the sync selection option in the default UV editing panel. I switched to another template and added the UV window manually and the sync selection button was there.