So, does the PHB need to explicitly state that at levels 1-4 rogues and fighters (but not barbarians or monks) are explicitly mundane. And further, that at level 5 (no earlier, no later) rogues and fighters become extraordinary. Also, some subclasses are magical (not extraordinary, at level 3) such as arcane tricksters and eldritch knights. At level 5, I’m not clear what happens: does the arcane trickster become extraordinary, or does it remain magical? Is the arcane trickster’s Evasion ability different from the thief’s? If the arcane trickster’s Evasion ability is magical, why can’t wizards and sorcerers access it?
The short answer is yes.
The longer answer is. The core rules emphasize the default setting assumes that levels 1-4 are Common, and levels 5-8 are Uncommon. These ubiquitous low-tier levels define the medievalesque setting. During these levels, the DM and players can refer to reallife experiences to guesstimate if a certain stunt should work.
Levels 9-12 exist but are Rare. They are anomalies and dont define the setting.
This "middle tier" of 9-12 represents a blurry line that mixes the peak of human possibility and subtle fantasy impossibility. The world record athletes mingle with action movie heroes − and wire-fu. This level is heightened and visually stunning, but still refers to the ordinary world, even if glancing across it impossibly.
During the high-tier levels, 13-16 and 17-20, the limitations of reallife are irrelevant. The constraints and assumptions about "medievalesque" are impossible to sustain under scrutiny. This level of play is the genre of superheroes. Nevertheless, levels 13-16 are Very Rare. There are not enough superheroes to disrupt the economy and ambitions of a medievalesque. While these anomalies exist, they cannot disrupt the overall feel of the default medievalesque setting.
The Legend tier of levels 17-20 are Uniques. Each individual is different. These are Primal beings and Celestial avatars, Archfeys and Archmages and legendary warriors who can singlehandedly defeat arrays of armies. This is the tier of Superman and Storm. Each person is unique.
The Commons and Uncommons are "Ordinary".
The Rares, Very-Rares, and Uniques are "Extraordinary".
Magic perfuses the default setting, but most of it is "Ordinary" magic. Even a
Fireball is comparable to a reallife grenade. While magic can and does do weird things, its effects are never of a magnitude that is sufficient to disrupt the medievalesque assumptions. Castle walls work as a defense, at least most of the time. Ordinary magic to bypass a castle wall has limitations of duration, resources, or vulnerabilities. Ordinary people cannot easily bypass a castle wall, including mages.
The Rare and rarer Extraordinary individuals are within the reach of "bounded accuracy", yet function at a magnitude that trivially bypasses the Ordinary world. The Rares inspire fear, respect, and fame. Ordinary people care deeply about what Extraordinary persons do with their powers.
If concluding that some fighter and rogue subclasses are mundane until level 4 and extraordinary afterwards allows you to make sense of the game, have at it. But don’t impose your conceptions on everyone else by saying they should be included in the PHB, particularly when they apply to a subset (2 of 13 classes) of a subset (less than half the subclasses) of a subset (only the races that aren’t innately magical).
So, "mundane" is a less useful term, when most of the world suffuses with magic. Ordinary (normal, typical) is a more helpful term.
