Update: Elspeth ban has been lifted, Lee Sharpe announced it via his twitter account linking to the following post on the Wizards Community site.

Due to a glitch with the card, Elspeth Knight-Errant has been temporarily banned on Magic Online. The announcement was originally posted on the Magic Online Wizards Community page. The glitch arose due to her ultimate ability and the emblem it creates for players.

The ban will be removed as soon as the glitch is repaired. In this way, Elspeth becomes the first planeswalker to be banned from any Magic format in any medium, physical or digital.