Diretide is an annual Halloween event that adds new themed game modes. According to the post, the developer underestimated how much players wanted the update. Compounded by "how busy everyone was with our next major update" and the work needed to resurrect the event, the team decided not to ship or inform players of that decision — which was "clearly a mistake."
"The rest of the update we've been working on is in final testing, and as each of us finish up our work on it, we've been jumping over to Diretide," the post reads. "We're pretty sure the update that we've been working on for a couple of months is going to be pretty significant. In addition, we've made a few changes to Diretide that we think makes it more fun than before.
"Paying attention to what is happening in the community around Dota 2 is one of the most important aspects of what we do at Valve. And while we are clearly not mistake-proof, hopefully it is useful for us to walk everyone through our process for recognizing and then fixing our mistakes."
You can read the full post on the Dota 2 blog.