The following items were previously undervalued in previous seasons of Teamfight Tactics.
Blue Buff
Since its introduction until the recent patch 11.24, Blue Buff has seen minimal alterations in Teamfight Tactics. Perhaps due to this, for the majority of season 5, Blue Buff was considered extremely low-tier as it was overshadowed by another energy item, Shojin Spear. In season 5, the Dark Sphere version of Blue Buff, named Deep Blue Buff, was preferred over the original.

However, in season 6, Blue Buff is emerging as a highly sought-after item. Firstly, we must acknowledge the stability of the Sorcerer team, with many carries effectively utilizing Blue Buff, from Malzahar and Lux to even Twisted Fate if desired. Furthermore, this item proves to be remarkably suitable for a potent 5-gold carry like Akali. Continuous energy regeneration enables Akali to maneuver the battlefield swiftly and easily dismantle enemy teams.
Morellonomicon
In season 5, most anti-healing champions possess abilities like damage mitigation (Knight), damage reduction (Cavalier), or high armor (Ironclad). Thus, Morellonomicon isn't an ideal choice to counter these champions during that time. Additionally, very few champions effectively utilize Morellonomicon in the same way Kennen did in season 4, leading to its relatively low evaluation among magic damage items.

This changed entirely in season 6 when Morellonomicon saw increased usage. The primary reason for this shift is the current abundance of healing, particularly from tech cores. Therefore, having an active counter item like Morellonomicon in your lineup becomes crucial. Furthermore, the prevalence of Giant units with high health demands players to possess a percentage health damage item.
Statikk Shiv
Unlike the aforementioned items, Statikk Shiv once held a top meta position at the beginning of season 5 when used in an Evil Demon hyper-roll team with Ziggs as the main carry. However, it quickly fell out of favor as the Evil Demon lineup was nerfed, and other reroll strategies became more dominant. Even if the Evil Demon lineup resurfaces, Statikk Shiv isn't prioritized anymore as players tend to focus on champions like Kled with items like Guinsoo's Rageblade and Runaan's Hurricane.

Entering season 6, Statikk Shiv made a spectacular comeback, particularly being utilized by Kog'Maw and Urgot. Kog'Maw alone deals significant magic damage through his abilities, while champions like Viktor and Lissandra from the Hextech tribe are formidable magic carries. Hence, Statikk Shiv becomes extremely popular when used by such powerful lineups.
