Twitter is now moving t.co to HTTPS only for new links from October 1, 2015 onwards. The HTTPS scheme helps Twitter securely transfer readers to the intended landing page.
One of the team members from the Twitter development team published a thread on the Twitter Community forum stating that the new company’s plan for HTTPS on new outbound links. So from October 1, 2015 onwards, if you share a link on Twitter, it will cloak in https://t.co. This HTTPS scheme allows Twitter to securely transfer readers to the intended landing page, even if the landing page is not an HTTPS link.
Going forward, this HTTPS change will increase the length of cloaked URLs by a single character. Another issue with this change is tracking referral traffic to non-HTTPS sites. Twitter explains that non-HTTPS sites may notice an apparent decrease in referral numbers from Twitter. Based on Twitter estimates, you may see a 10% drop in traffic attribution as a result of this HTTPS change. Also, the percentage will decrease over time as users update their browsers to the latest versions supporting to the referrer policy.