New mechanics are tried, tested, and put into Magic: The Gathering all the time. While many don’t make it past the drawing board, a select few manage to be popular enough to make it into the developing process only to wind up fizzling out. One of the closest of these "almost made-its" mechanics was simply known by the ominous word "‘Forbidden."
During the 2008 Magic World Championship in Memphis, Magic creator Richard Garfield had a problem. He had created a spellslinging deck for some duels but kept getting a bunch of cards that weren’t helping him out. Calling them "Gunk" cards, he tried to think of a way so that they wouldn’t be killed off outright (graveyard), wouldn’t keep coming up (library), and couldn’t go to his opponent. But what about exile?
After getting back from Memphis, the new mechanic of Forbidden was born. It allowed card to go straight into exile. Well, sort of.
The idea went down to Mark Rosewater and the R&D team at WotC HQ. As it went through the rounds, one designer, Brian Tinsman, took a real hold of it and tested it even more for the expansion he was in charge of: Avacyn Restored. It even had a few storyline points attached to it, such as unlocking the secrets of the Helvault. With many a designer and higher-ups pulling for it, it looked like Forbidden was ready for prime time.
Forbidden was soon being expanded so cards didn’t just go straight to exile. Some cards could be shuffled through while others were allowed to go to the library. The cards created a sense of urgency to use around them because of what Forbidden could do to cards if the player played through regularly. It was hectic….a little too hectic, in fact.
By the time it was playtested in "devign" (which is what WotC calls the period between design and development) it was found to simply not work. Despite Tinsman trying over and over again with the creative team it was clear that it would be difficult to put in.
To meet the 2012 Avacyn Restored deadline, the Miracle mechanic replaced Forbidden. Tinsman still tried periodically to put it in and Rosewater to this day has been adamant about it working. But. since 2012. it hasn't gone back up to R&D.
We may see the mechanic in the future, but for now Forbidden remains, well, forbidden.