The @ Mention feature is available in Planner 1, which is $10/month per user. It's not that they haven't already done it, it's that they want you to pay for it. Also, I found this out only when I had the most rudest experience from Microsoft. One of their customer service reps put me on hold without telling me when I asked what version of Planner offers @ mention feature. I had to call back. And I called because I couldn't get a straight answer out of the online sale rep. I could go on, but the experience was abysmal. Even speak with the person on the phone I could tell they were reading off scripts or something. That gave me zero confidence regarding getting support in the future. Either way, the feature is available, Microsoft just wants more money is all.