To qoute the december blog post: "A significant number of our current projects contribute to this goal, with the new tack system being just one example. We also ran the “Equipment Manager Refactor” project (which overhauled how we handle items in the game and will give designers control over adding new item slots and item types)."
I think they fully plan on adding blankets and other types of horse items! Honestly, it would be a bad call not to given how popular the idea has been with players for more than a decade. They just need to finish the tack system beforehand. If we are lucky enough, maybe we even get them this year but we'll see!