Possession has been banned on Dominion Online, and you will see this go into effect in a couple of days. You can still create a table and select Possession to still play with it. Coming soon will be an option to ban up to 5 cards of your choosing, at which point, Possession will no longer be banned, but you will have the option to ban it.
Context can be found here.
Dominion Strategy 
What happened? That card has been around for a long time.
Context, which we’ll add to the main post: http://forum.dominionstrategy.com/index.php?topic=17857.0
What happened? That card has been around for a long time!