In November 2021, Games Workshop introduced the Warhammer 40k Dataslate, a collection of small incremental changes for competitive and official play of the dark sci-fi miniatures game. These are usually done to address potential issues or oversights in the game's overall balance when it comes to high-end play at official tournaments, like the recently concluded events at AdeptiCon. Overall, the dataslate addresses mostly new units introduced recently for the various armies, including the Aeldari Harlequin units.
According to their official Warhammer-Community post, Games Workshop outlined the changes coming in the latest Warhammer 40k Dataslate. The very first army united cited by Games Workshop were the recently updated Harlequin units, servants of The Laughing God. Thanks to recent major changes that came as part of the new Aeldari Codex, harlequin units ended up seeing a lot of top-level play according to the latest Metawatch. Because of this, the Harlequin's Mirror Architect and Favour of Cegorach abilities have been adjusted. Now, these abilities, "need more care to use due to their powerful nature."
But the Harlequins aren't the only units getting adjustments in this Warhammer 40k Dataslate. The Adeptus Custodes' Objective Secured ability has now been changed so it only applies to Troops, allowing larger units more reign of the battlefield and avoiding more aggressive lockdowns. The Tau's Broadside Battlesuit units have had the Core keyword removed, as well as the removal of specific charge-blunting rules no longer stacking.
The latest Warhammer 40k Dataslate also includes more sweeping changes for other units. Various armies associated with the Imperium of Man, including the Adeptus Astartes, Sanctic Astartes, Adepta Sororitas, and Heretic Astartes have all received a new ability called Armour of Contempt. Simply put it reduces the armor penetration damage of attacks targeting these units by one. This was done in response to how more lethal Warhammer 40k has become recently in order to keep these armies viable. Next, there is the tweaking of the ability Indirect Fire. Now, anything shooting at a unit that doesn't have line of sight has its Ballistics Skill reduced by one while the targeted unit gets a bonus to its saving throw. The only exception to this rule are the Astra Militarium weapons given their whole appeal is that they're giant guns that make things go boom.
There are plenty more changes packed within the Warhammer 40k Dataslate, and for those who play the game competitively should look it over as soon as they can.