Sometimes Dungeons & Dragons players ask to do things that the rules don’t handle—and not just because no roleplaying game’s rules can cover everything. The game omits the added rules because they would run against D&D’s design approach. Often, past editions of the game even included these extra rules, but fifth edition’s more consistent design forced them out.
What actions aren’t covered by the D&D rules because they defy the game’s design choices?
1. Readying an action outside of combat.
In the early days of D&D, players liked saying, “I ready my sword” or “I nock an arrow.” Back then, initiative ran by house rules and DM whim, so this sort of declaration might win an edge. In an example of playing D&D that Gary Gygax wrote for the Europa zine in 1976, the DM grants the party +1 to a d6 initiative roll for being prepared. Only Unearthed Arcana (1985) actually put a benefit for readiness into print. “A bow specialist who begins the round with arrow nocked, shaft drawn, and target in sight is entitled to loose that arrow prior to any initiative check.” (See For 10 Years D&D Suffered From an Unplayable Initiative System. Blame the Game’s Wargaming Roots.)
Despite the lack of rules benefits, such declarations might prevent an adversarial dungeon master from deciding that because you never said that your sword was drawn, a fight caught you unprepared. In those days, many gamers saw thwarting and punishing players as part of the DM’s role. That attitude has fallen from favor. Nowadays, even though you never mention that your character started the day by putting on pants, we still assume pants.
The old spirit of readiness continues today. In a Sage Advice segment, D&D lead rules designer Jeremy Crawford says, “People often want to ready actions before combat has even started.”
Why “readying” does nothing. Jeremy says, “The ready action is an action you take in combat, so there’s really no such thing as readying before combat has started.”
What a DM should say. Maybe nothing. “I rarely correct them,” Jeremy says. He interprets the ready request as the player signaling their intent to stay alert. “Usually what that means is they won’t be surprised at my table. So even though I won’t have the mechanics of the ready action play out, I will still reward them for thinking in advance and signaling intent.”
To more rules-oriented players say, “In D&D, everyone who isn’t surprised starts a fight ready. Initiative lets us decide who among the ready combatants goes first.” (Surprised combatants also have a place in initiative, but they take no actions while surprised.) For more on what to tell players about initiative, see What to do when a player interrupts a role-playing scene to start a battle.
2. Called shots.
Sometime in every player’s D&D career, they get the idea of skipping the process of hacking through all a creature’s hit points by simply chopping off their weapon hand or blinding them with a blow to the eyes. Earlier editions of D&D termed attacks that aimed for a specific body part “called shots” and the game once included rules for such strikes. No more.
Why the rules don’t include called shots. The fifth-edition Player’s Handbook explains, “Hit points represent a combination of physical and mental durability, the will to live, and luck.” That’s a lot of possibilities. The game rules rely on this vagueness to allow characters to regain all their hit points after a short rest rather than a long hospital stay.
The moment characters start attempting to gouge their foes’ eyes out and villains return the favor, the game loses the useful abstraction of hit points. Also, if aiming for a particular body part proved more effective, why would anyone bother with regular attacks? The second-edition Dungeon Master’s Guide allowed called shots, but explained, “Because the AD&D game uses a generalized system for damage, called shots cannot be used to accomplish certain things. Against a creature, a called shot will only cause the normal amount of damage allowed the weapon. Attempts to blind, cripple, or maim will not succeed.”
What a DM should say. “Your characters are experts at combat. With each attack, they use their skills to find the best opportunity to land a blow that deals the most damage and that offers the best chance of taking your foe out of the fight.”
3. Delaying an action.
In third and fourth edition, players could delay their actions to later in the initiative count. This helped players coordinate actions with other players using an advanced strategy called you set ’em up and I’ll knock ’em down. Players who remember the flexibility of delaying still ask for it.
Why the rules don’t include delaying. The fifth-edition designers chose to eliminate delaying to simplify the rules and speed play.
Many spell durations and combat effects last until the beginnings and endings of turns. An option to delay complicates the rules for such effects and the bookkeeping needed to track them. “Simply by changing when your turn happens, you could change the length of certain spells,” explains the Sage Advice Compendium. “The way to guard against such abuse would be to create a set of additional rules that would limit your ability to change durations. The net effect? More complexity would be added to the game, and with more complexity, there is greater potential for slower play.”
An option to delay encourages players to analyze and discuss the optimal order for their turns during every round. “Multiply that extra analysis by the number of characters and monsters in a combat, and you have the potential for many slow-downs in play.”
What a DM should say. “Everyone in a fight acts at once. We just have turns to make some sense of that activity. If you delay, you do nothing while everyone else acts. At best, you can start an action and attempt to time it so that it finishes right after something else happens. That’s called readying an action.”
Even though the megadungeons under Greyhawk and Blackmoor became the foundation of Dungeons & Dragons, such dungeons rarely see play anymore. Why not?
So rather than attempting to capture Greyhawk Castle, Gary opted to publish adventures that he had created for D&D tournaments at conventions. For instance, the official D&D tournament at Origins ’78 ran the 
The Ruins of Undermountain
3. Play styles expanded. Sometime in the middle of the 70s, for the first time ever, a party of adventurers visiting the inn met a hooded stranger with a job that needed doing. D&D expanded beyond a series of dungeon expeditions aimed at claiming treasure. Players began to favor games that mixed action with story. 
The original list revealed that fifth-edition versions of the 4 spells all added changes that turned them from forgettable to powerful—and aggravating. The 3 spells on this new list all started powerful, even among 6th- and 7th-level peers. Two of the spells’ 1st-edition descriptions start, “This powerful spell…”