In reply to Peter Müller.

It looks like the option has been restored, along with other link related items in the 2.7 release:

Several updates to link insertion interface:
*Restore the “Open in new window” setting.
*Remove the Unlink button. Instead, links can be removed by toggling off the Link button in the formatting toolbar.
*Move link settings to the left.
*Update suggested links dropdown design.
*Allow UI to expand to fit long URLs when not in editing mode.
*Improve visibility of insertion UI when selecting a link

Yea!