The ability to delay actions in combat in Dungeons & Dragons 5e isn't actually allowed in the rules. The ability to delay an action is tied to DnD's Initiative system in combat, which sets every combatant's turn at the start of a battle. Once an Initiative score is rolled, then it's set for the rest of the encounter, even if a character is KO'd by an attack and revived mid-battle.
There are many DnD campaigns that use homebrew rules, as every group has its own take on what constitutes as a fun addition to the rules, or is something that balances something that is too strong or weak. In fact, it's harder to find a group that fully embraces the rules as written and doesn't add their own twist. There are some DnD rules that fans don't even realize that they've been homebrewing, either because they were in a previous edition and assumed it made the jump to DnD 5e, or they made so much sense that the players never questioned whether they were printed in the Player's Handbook.
Older Editions Of DnD Let Players Delay Their Action
In the D&D 3e rules, a player had the ability to use the Delay or Ready actions in combat. The Delay action allowed the player to hold what they were doing until further down the Initiative queue, allowing them to act later in the round. This allowed them to better respond to events happening in that turn, such as waiting to see how the other characters damaged the enemies, allowing them to prioritize their attacks to finish off wounded foes. A cleric player might also want to forego the ability to perform an attack so that they could wait and see who needs healing. This means that they can spend their turn patching up a party member who takes a lot of damage, which could be important later on. Delaying is a smart move in DnD's turn-based combat system, but it has a downside, however, as it sets the character's action to the new, lower score. This means that there is a risk involved, as they're potentially losing the ability to perform an action ahead of the enemy.
The Ready action was different from Delay, as it allowed a player to have their character respond to an enemy's action. An example of this could involve a battle with an enemy spellcaster, with the player using Ready to perform an attack as soon as they start casting a spell. This would have the added benefit of potentially interrupting the spell, causing it to fizzle out. Like the Delay action, the Ready action also pushed the character's Initititave score to a lower value.
D&D 5e Removed The Ability To Delay Actions
The ability to Delay an action is gone, as it's one of the DnD rules that don't appear in 5e. The reason why the ability to Delay is gone was actually explained in the Sage Advice Compendium on D&D Beyond. The reasons given for the change were that the ability to push back the player's turn can slow down combat, especially if it extends to the party, as well as the monsters. DnD 5e also discourages the ability of a character to change their position in the Initiative order, as it makes it harder for the DM to keep track of the Initiative for every combatant, especially in situations where there are lots of characters on the field.
The ability to Delay would also impact spell durations, most notably the ones that specify that they last for a single turn. Despite these issues, the Delay action is one of DnD 5e's popular house rules and DMs will allow it, as it makes sense for a character to wait for the right time to strike, rather than wasting an action using something like Dodge.
The Ready action does exist in DnD 5e, but it works differently this time around. If the player readies an action, they dedicate their Reaction to firing when a specific trigger is met, such as when an enemy steps into a room. This doesn't change their place in the Initiative order and if the trigger doesn't go off, then they have wasted their round. A player can ready a spell as part of the Ready action, but this requires them to concentrate on the spell, allowing it to potentially be broken if they take damage before the trigger goes off.
One D&D Could Potentially Bring Action Delaying Back
It's not the end for Delay action in DnD 5e, as there's a chance that it could reappear in the future. The current version of DnD is evolving, as DnD 5e is being upgraded in 2024, with new versions of the core rulebooks that are being released as part of the 50th anniversary celebrations of the game. This means that there will be a new incarnation of the Player's Handbook, with new rules that are said to be backward compatible with the existing DnD 5e ruleset.
Fans are already experiencing the changes that will be introduced in the new Player's Handbook, as One D&D is releasing playtest material for the upcoming rules additions. These include adding brand new playable races, giving every character a Feat at first level, changing the benefit received at level 20, and even bringing AD&D class groups to DnD 5e. One D&D has the chance to introduce a new version of the Delay action rules to DnD 5e's combat system, one that isn't as disruptive as the one used in DnD 3e.
One D&D has been experimenting with different ideas and giving players the chance to try them out before they're printed a book. These have included major changes to the critical hit rules for monsters and spells, which were quickly rescinded in the next update. One D&D might be the perfect venue to test the Delay rules, and if they don't work, then they can safely be ignored in the upcoming Dungeons & Dragons rulebooks.
Source: D&D Beyond