Late last year, Twitter announced that it would be making t.co links longer. What this means for users is that they get two characters less to write their tweets if they include a link in the message.
It may not seem like much, but considering you only get 140 characters in total, each one counts.
The change is now in effect, if you add a link to your tweet you will only get 118 characters for the rest of the message, down from 120 as before.
In fact, you'll get only 117 characters if it's a HTTPS link, you can thank the extra “s” for that. For several years now, Twitter has been shortening and wrapping any link on the site with its t.co URL shortener.
Twitter never really said why it's implementing this change; the existing length was enough to encode trillions of links.