Issue with options in ICME file uploader missing
I have the same issue. This is a big headache.
We want to make all off-site (external) links have target = _blank.

Solution: The CKeditor Anchor Link module, anchor_link, adds the other link properties (as it existed in Drupal 7). You have to edit the text formats and include the new buttons that anchor_link creates. However, these buttons are identical to the ones created by the Link module (in core D8), so make sure to remove them and drag the new ones (from anchor_link) in.