In a stunning announcement Felidar Guardian has been added to the Standard Banned List effective April 28 as an addendum to Monday's Banned List update. The ban will be implemented on Magic Online tomorrow, April 27, with a special downtime. Participants in Standard Leagues will have their entry fee refunded.

From Aaron Forsythe:

Why are we making this call now and why didn't we make it in our regular B&R announcement on Monday? The answer is data. We knew going into Monday that the Saheeli-Felidar combo was a significant issue for Standard and were watching it closely. Our rationale for waiting was to make sure we only needed to take one and only one action to correct the Standard environment. Our plan was to monitor early play behavior and pro competition and make a call following Pro Tour Amonkhet.

Since the initial announcement, for the first time ever, we pre-released a new card set on Magic Online before the formal release date. This happened on Monday. What we expected to take a few weeks to understand has ended up taking two days to form a clear picture of a metagame unbalanced by the Copy Cat combo, as even its natural predator, Mardu Vehicles, fell behind. Couple this with consensus among a wide sampling of pros and feedback (and pizza) from our community and we decided to take action.

Saheeli-Felidar's win-loss ratio and metagame share has actually increased since the release of Amonkhet. In Magic Online Standard Leagues since Monday, Saheeli combo has made up approximately 40% of 5-0 and 4-1 decklists—up from prior to Amonkhet's release. While we never take decisions like this lightly and recognize this is a change from the norm, when a plurality of the data points in a clear direction, we will take action.

We also understand we shouldn't let combos like Saheeli-Felidar get out the door in the first place. For that we take ownership and are making changes to try to prevent this from happening again. But our highest priority is keeping Magic fun and enjoyable for our players. We believe this banning coupled with a number of internal testing process improvements will be significant steps toward making Standard the fun, dynamic format we all want it to be in perpetuity.

This is an unexpected turn of events for many reasons. The first is the timing, so shortly after the original announcement, and that a few short days of data could have that much impact. Also of note is the public outcry and its potential to sway the powers that be when backed up with evidence. We saw this recently with the outcry regarding Modern Masters 2017 commons being excluded from Pauper online and the the public reaction forcing action. As Forsythe notes, the hope is that these decisions will not have to be made in the future and that not only will Wizards get it right the first time, but also not release unhealthy cards and combinations into the metagame.