Here's another working link, from Gary at the View from the Wing blog:
https://www.barclaycardus.com/apply/Landing.action?campaignId=2062&cellNumber=13&referrerid=PTREMREMIND. While it is in the same/similar format as personal referral emails, this one seems more generic. Other referral links are pretty obviously tied to a specific member with an 8-digit tracking number. I think that matters, because personal referrals have a limit of 5 iirc, so this one would not seem to be subject to that limitation. I would think that even if a link's five referrals have been used, that the link would still work and you could apply, but the referrer would just not get the referral bonus. But, I don't know that's the case.
The official policy of Barclays is that you can have only one of these cards. But, their computers think otherwise
I have two (got the first in Aug 14, the second in late Jan 15) and am considering going for number 3 in a few days before they're gone for good. These cards will be converted into Aviator cards, which will not be issued to new customers. So, this does truly seem to be the last gasp. Also, getting one of these has no impact on your ability to get an AAdvantage card from Citibank (other than the typical credit inquiries/history). So, you could pretty easily pick up 100k AAdvantage miles for $5k spend and $89.
In the interest of FD, I have no stake in this link, nor the VFTW blog, just offering this info as a PSA.