maxloader.nl

The end has apparently been jerf'd (Ace's term; a portmanteau of Jeb and nerf). In the latest snapshot, you can no longer build outside of the mainland in the End (according to Docm77, who has been very reliable in the past). Also known as Jeb jerf'd the Ender Ender. Currently existing structures will apparently still exist, however, so if you want to build anything, better do it before the jerf is officially released! (Or bug jeb till he unjerfs it)

I have proof that this is just a glitch now, I'll post pictures in a few minutes.

This is like 100 blocks away from the edge of the mainland:

This is the mainland:

As you can see, the limit is a bug, I was able to temporarily fix it by relogging whenever the "barrier" seemed to have been reached.
Also I was able to ban myself from singleplayer: