The next Dungeons & Dragons are intended to be among the elite, which means that their classes would be overpowered if used to stat out everyone living in a city.
The third edition of Dungeons & Dragons introduced a compromise in the form of the NPC class rules. The Dungeon Master's Guide contained five classes that were meant for NPCs. The adept was used for untrained or unrefined spellcasters, the aristocrat was used for people of noble birth, the commoner was meant for the unwashed massed, the expert was meant for craftsmen and professionals, and the warrior was meant for town guards and common soldiers.
The fifth edition of Dungeons & Dragons has rules Monster Manual, but the NPC class rules will be making a comeback in the Tasha's Cauldron of Everything rulebook. There will be an entire section dedicated to sidekicks, who are basic characters that are easy to create and even easier to run. The intention is for sidekicks to be used as entry-level characters for younger players who are still getting to grips with the rules, or for extra characters that can be run by players or the DM if the group needs extra on the fly, such as when a player cannot make the game at the last minute.
How Dungeons & Dragons Utilizes Sidekicks & 3rd Edition Rules
It was confirmed during a recent press event for Tasha's Cauldron of Everything that the three classes used by sidekick characters are the commoner, expert, and spellcaster. These are new incarnations of the old NPC classes from third edition, as they use simplified rules for established classes. These can be applied to sidekick characters, but they can also be given story-relevant NPCs at short notice. A good dungeon master could easily create a couple of basic stat blocks for each of these classes and save them for whenever they need to quickly put an NPC on the battlefield.
The NPC class rules were a great idea when they were added to third edition of Dungeons & Dragons, and it's surprising that it took this long for them to be implemented in fifth edition. The sidekick idea in Tasha's Cauldron of Everything as a whole is a great concept, and it will act as a great learning tool for new players as they are gradually introduced to the rules of the game.
Tasha's Cauldron of Everything will be released on November 17, 2020.
Source: Wizards of the Coast.