Currently all links that open in a new tab display an arrow icon. However there are some external sites that are considered internal. For example domains like act.greenpeace.org or donate.greenpeace.org or others that sometimes even keep the same look and feel of P4.

PLANET-4334 is intending to remove the arrow icon and "opening in a new tab" behaviour whenever the link redirects to a URL that is part of an "Exceptions list". Please review this behaviour and flag if there are any issues on UX.

Ask ltiralon if you need more info.