Valve has officially scrapped the event for 2015, opting instead to have a larger presence at GDC 2015. When asked about the event, a Valve representative issued the following statement.
"Steam Dev Days was a great way to brief a large number of Steam developers. This year our focus will be a bit different, so we are planning a larger than usual presence at GDC. So, there will not be a Steam Dev Days this year, but we will certainly consider doing it again in the future."
I suppose this really shouldn't come as any surprise to anybody.
(via)