https://blogs.magicjudges.org/telliott/2018/10/01/policy-changes-for-guilds-of-ravnica/

4.8 Reversing Decisions

Players are expected to consider their options before taking an action and players are not usually allowed to take back an action that has been communicated to their opponent, either verbally or physically.

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Sometimes, a player will realize that they have made a wrong decision after making a play. If that player has not gained any information since taking the action and they wish to make a different decision, a judge may allow that player to change their mind. Judges must carefully consider whether the player has gained information since making the play that might have affected the decision; in particular, players may not try to use opponent reactions (or lack thereof) to see if they should modify actions they committed to. If the judge cannot be sure no information was gained, they should not allow the decision to be changed.

Examples

1. A player plays an Island and, before anything else happens, says “Sorry, I meant to play a Swamp.”
2. A player says “No blocks” immediately followed by “Wait, no, I block with this creature.”
3. A player says “Go. Wait, land, go.”

This does not seem like a great change ...

@themonadnomad I strongly agree, it will hurt game play. Even MTGO does not allow take backs

I think it's still judges discretion, which was what it was before hand, if unofficially.

For example, this protects against "Fetch, Get Tundra", then, while still shuffling, and literally not having finished resolved the action, saying, "Sorry, fetch Basic Island"

I think the provided examples aren't even the most obvious cases--e.g. "Play Swamp" and put down an Island, and backing up to "Sorry, wrong card", or after some long sequence, opponent says "I'd like to go to attacks," and "sure, oh, sorry, I have effects before you attack" (before the opponent starts tapping creatures, in one mostly complete thought).

I think it's not going to be a functional change, but it is giving Judges leeway within the rules to do common sense decisions.

@moorebrother1

MTGO has the undo button for mana sources, which is technically a take back.

This is a great decision as written. High pressure situations really shouldn't come down to nervous miscommunications or issues with language barriers.