It might seem weird to have Ashiok, Dream Render in the sideboard rather than in the main deck since on its face, it looks like one of the best mill cards in Standard, potentially milling 20 cards all by itself for just three mana
See where this goes

Sort Rarity Card Type Caterpie #1Common $0.25 Metapod #2Uncommon $0.21 Butterfree #3Rare $0.54 Oddish #4Common $0.25 Gloom #5Uncommon $0.27 Vileplume #6Holo Rare $0.79 Tangela #7Common $0.18 Tangrowth #8Rare $0.22 Ledyba #9Common $0.20 Ledian #10Rare $0.31 Heracross #11Rare $0.30 Pansage #12Common $0.12 Simisage #13Uncommon $0.17 Dewpider #14Common $0.05 Araquanid #15Rare $0.23 Wimpod #16Common $0.17 Golisopod GX #17Ultra Rare $1.59 Charmander #18Common $0.32 Charmeleon #19Uncommon $0.29 Charizard GX #20Ultra Rare $24.66 Ho-Oh GX #21Ultra Rare $5.54 Pansear #22Common $0.07 Simisear #23Uncommon $0.03 Heatmor #24Uncommon $0.12 Salazzle GX #25Ultra Rare $2.06 Turtonator #26Rare $0.24 Alolan Vulpix #27Common $0.35 Alolan Ninetales #28Rare $1.90 Horsea #29Common $0.24 Seadra #30Uncommon $0.15 Kingdra #31Holo Rare $0.63 Magikarp #32Common $0.34 Gyarados #33Holo Rare $2.57 Marill #34Common $0.68 Azumarill #35Rare $0.29 Panpour #36Common $0.12 Simipour #37Uncommon $0.18 Bruxish #38Rare $0.21 Tapu Fini GX #39Ultra Rare $2.45 Pikachu #40Common $1.12 Raichu #41Holo Rare $1.20 Electabuzz #42Uncommon $0.15 Electivire #43Rare $0.29 Tynamo #44Common $0.50 Eelektrik #45Uncommon $0.14 Eelektross #46Rare $0.15 Togedemaru #47Common $0.17 Slowking #48Rare $0.51 Wobbuffet #49Uncommon $0.22 Seviper #50Uncommon $0.19 Duskull #51Common $0.19 Dusclops #52Uncommon $0.05 Dusknoir #53Holo Rare $0.40 Croagunk #54Common $0.18 Toxicroak #55Rare $0.27 Venipede #56Common $0.14 Whirlipede #57Uncommon $0.05 Scolipede #58Rare $0.27 Espurr #59Common $0.24 Meowstic #60Rare $0.39 Charizard GX (Secret) #150Rainbow Rare $607.29 Fairy Energy (Secret) #169Secret Rare $88.09 Wicke (Full Art) #147Ultra Rare $62.97 Darkness Energy (Secret) #168Secret Rare $53.74 Acerola (Full Art) #142Ultra Rare $51.14 Gardevoir GX (Secret) #159Rainbow Rare $51.09 Darkrai GX (Secret) #158Rainbow Rare $50.45 Machamp GX (Secret) #154Rainbow Rare $38.50 Darkrai GX (Full Art) #139Ultra Rare $32.23 Marshadow GX (Secret) #156Rainbow Rare $29.63

A Play Booster is guaranteed to contain at least one rare or mythic rare, but it can contain more
A few years ago, the loophole was closed when the policy was expanded to include never printing these cards in any shape or form that is tournament legal (promos, duel decks, archenemy, etc.) To this day, Wizards hasnt shown any sign of changing the policy, with staunch promises from several high-level employees that the reserved list is here to stay
The curse of the Wicked Slumber roils through the fairy tale lands of Eldraine, and while Wil and Rowan do their best to save their kingdom and perhaps their plane, we'll be taking a look at how the new multicolored cards seem poised to shake up formats with the upcoming release