This is a function that we use extensively across our pages and it seems to have been removed recently. Some context to our requirement:
In the landing page editor, if a form follow-up page is set using Follow-up type > Landing Page > ThankYou Page, or Follow-up Type > External URL > Absolute Path URL when the form is used Google Analytics will reset the submitter's session, removing any referral or source/medium information. The session reset may not affect all users, it's a side-effect of the type of cross-domain tracking we've had to use to maintain sessions between Marketo landing pages and our main websites.
By setting a thank you page with a relative path (Follow-up type > External URL > Relative Path URL) we are able to prevent Google Analytics from restarting a submitter's session. This was possible until not long ago so we are hoping relative paths can be allowed again. Our active pages are all using relative URLs right now, it's only when we try to change them do we receive an error.
Any other ideas on the above problem would also be much appreciated.