Source: https://ygorganization.com/whyarentyoubuyingddcrowsrightnow/

You didn’t actually think that we’d get a new Master Rule with just Extra Deck changes right?

The following will take effect in the OCG starting on April 1st 2020.

Regarding the Activation of Monster Effects for Monsters Returned to the Deck or the Extra Deck Face-Down
-Unless it is specifically referenced in the text, monsters that are returned to the Deck and the Extra Deck face-down cannot activate their effects (this includes mandatory effects).
Example of Old Way of Resolving: If “Elemental HERO Absolute Zero” is returned to the Extra Deck by “Compulsory Evacuation Device”, its effect to destroy all monsters your opponent controls will activate.
Example of New Way of Resolving: If “Elemental HERO Absolute Zero” is returned to the Extra Deck by “Compulsory Evacuation Device”, its effect to destroy all monsters your opponent controls will not activate.

Regarding the Activation of Card Effects that Move Away from Their Activation Location Before Their Effects Activated
-In card texts, cards refer to their effect activation requirement as well as their effect activation location. (For example, if it is an effect that activates when the monster itself is Normal Summoned, it activates on the field, if it is a Trap Card whose effect is activated if it is sent to the GY, it activates in the GY.) If a card moves location from its activation location to a different location before its effect activated, its effect cannot be activated (including mandatory effects).
Example of Old Way of Resolving: “Sangan” is Tributed to activate “Crush Card Virus”, and “D.D. Crow” is activated in a Chain to banish the “Sangan” from the GY. “Sangan” activates its effect after this chain is done resolving. (In this case, “Sangan” is treated as activating its effect in the GY.)
Example of New Way of Resolving: “Sangan” is Tributed to activate “Crush Card Virus”, and “D.D. Crow” is activated in a Chain to banish the “Sangan” from the GY. “Sangan” does not activate its effect after the Chain is done resolving.

Regarding the Normal or Special Summoning of a Monster in Relations to Card Effects that Mention “During the turn you activate this card/effect”
-Some cards will have text such as “During the turn you activate this effect, you cannot Special Summon monsters, except [blank] monsters” or “During the turn you activate this card, you cannot Summon monsters, except with this card’s effect”, which have activation requirements for whether or not you Summoned certain monsters during the turn. Cards with this text will now check whether or not the Summoning of the monster described in the text was successful for their activation requirements.
Example of Old Way of Resolving: You cannot activate “Rage with Eyes of Blue” during the turn the Normal Summon of your “Maiden with Eyes of Blue” was negated by “Solemn Judgment”.
Example of New Way of Resolving: You can activate “Rage with Eyes of Blue” during the turn the Normal Summon of your “Maiden with Eyes of Blue” was negated by “Solemn Judgment”.

Regarding the Special Summoning of Monsters that Mention They “Can only be Special Summoned once per turn”
-Certain cards have the text “You can only Special Summon [blank] once per turn” which means that you can only Special Summon card(s) with that name 1 time during this entire turn. Cards with this text will now only count the Summoning if the Summoning of a monster with that name was successful during the turn.
Example of Old Way of Resolving: If your “Hi-Speedroid Chanbara” Synchro Summon was negated by “Solemn Judgment” during this turn, you cannot try to Synchro Summon “Hi-Speedroid Chanbara” or try to use any other type of card effects to Special Summon it for the rest of the turn.
Example of New Way of Resolving: If your “Hi-Speedroid Chanbara” Synchro Summon was negated by “Solemn Judgment” during this turn, you can Synchro Summon “Hi-Speedroid Chanbara” or try to use any other type of card effects to Special Summon it for the rest of the turn.

Regarding Continuous Trap Cards that Special Summons Themselves to the Monster Zone by Their Own Effects
-Continuous Trap Cards such as “Embodiment of Apophis” which has the effect “Special Summon this card as a Normal Monster (Reptile-Type/EARTH/Level 4/ATK 1600/DEF 1800). (This card is also still a Trap Card.)” that can Special Summon themselves to the Monster Zone will now free up the Spell & Trap Zone they were placed in before.
Example of Old Way of Resolving: You have 5 cards Set in your Spell & Trap Zone, you activate 1 of them, which is “Embodiment of Apophis” and Special Summons it to the Monster Zone. The Spell & Trap Zone that “Embodiment of Apophis” was in before cannot be used, so you cannot Set a new card in your Spell & Trap Zones.
Example of New Way of Resolving: You have 5 cards Set in your Spell & Trap Zone, you activate 1 of them, which is “Embodiment of Apophis” and Special Summons it to the Monster Zone. Because the card of “Embodiment of Apophis”is now in the Monster Zone, the Spell & Trap Zone it was in before is now empty and available for use, therefore you can Set a new Spell/Trap Card into the Spell & Trap Zone it was in before.

Source: https://www.yugioh-card.com/japan/notice/revision/#processing