As long as there has been Magic: The Gathering cards, there have been abilities. These keyword abilities sum up what action is being taken, and thus, are generally used a lot. To date, only five have been totally unique and limited to a single card. And, to date, only one of those looks to have a chance of coming back. That ability? Frenzy.
Frenzy was a triggered ability that gives a boosted +N/+0 ability to the creature in play if not blocked. Mark Rosewater specifically crafted the ability to encourage blocking because, if that creature isn't blocked, it can get really big really fast. It's the whole point of it as block is usually an underused mechanic.
We have to mention it was specifically crafted, too, because it didn't come up alone out of the blue. Five new abilities for five new cards were made all at once in 2007 for the new slivers in the Future Sight set. Five slivers, a hive mind creatures inspired by the Plague Rats card from Alpha, quickly took off as a creature type a decade before, but also came with some sort of unique card. The five slivers in Future Sight simply carried that tradition on. For frenzy, it was put on the creatively named card, Frenzy Sliver.
Most of the abilities introduced there, like fateseal, have already been called out on never returning again. Frenzy, however, has been brought up time and time again to return. Rosewater himself loves the ability and keeps trying to bring it back, but Play Design isn't as big a fan, saying that it's not really all that fun. And thus, why it hasn't returned.. Not in black-bordered Magic, anyhow.
It was featured in an Unstable card in 2017, Garbage Elemental, but that doesn't really count and was more like a thrown bone to Rosewater to bring the mechanic back ten years after the last time it was used. But that hasn't really dampened his and others hopes that it will be back - recently, he even said that, out of all the abilities out there that have been only on one card, frenzy is the one most likely to come back. As the ability would work best with slivers, it's most likely dependent on if slivers make a huge push in future decks.
But still, we have seen the return of long lost creatures and moves and abilities before. Frenzy shouldn't be that far off at returning.