I don't read any Adventure Paths or Adventures or anything kindred. The overwhelming majority of them (but not all of them), regardless of D&D edition or system, are examples of the worst sort of GMing possible. They typically preconcieve endpoints in immediate scenarios and long-term campaigns and in-so-doing instruct and/or encourage GMs to deploy Force to achieve outcomes. Further, the writing is typically either terrible for the system or incoherent with the primary themes of the system.
Scenario Packs or premises, sure. Adventures and APs. Absolutely not.
What I do read is the GMing advice/principles and the game's play agenda.
This is akin to choosing the sources that support your assertion and choosing to ignore those that don't. I'm not sure this does anything except strengthen the observatrion that this isn't an inherent 4e thing but instead fiction that an individual DM can choose to overlay their camapign with. Or that it is inherent and being consistent in it's application just sucked when it came to official sources out side the rulebooks.
D&D 4e's DMGs are very clear on what Epic Tier is about and what you should be doing as GM:
1) "...characters have truly superheroic capabilities, and their deeds and adventures are the stuff of legend. Ordinary people can hardly dream of such height's of power."
From the 5e DMG... Levels 17-20 Masters of the World
"...characters have superheroic capabilities, and their deeds and adventures are stuff of legend. Ordinary people can hardly dream if such heights of of power...or such terrible dangers."
It's nearly the same description word for word. The only difference here I would say is that level 20 is considered the pinnacle and when true Epic (as I understand it in 4e) takes place in 5e.
2) Regarding content, the instruction on Epic Tier tells GMs (as it does with Paragon Paths et al) to take cues from the players' Epic Destinies.
Relating to (2), here are a smattering of Epic Utilities that do not have any magic power source keywords (therefore, they aren't "magic-derived" in the D&D technical sense):
snipped for brevity...
And 5e's description directs the DM to use Epic Boons to represent the powers and abilities of Epic tier/mythical PC's. Referencing the section on Epic boons one is told what types of things these abilities are supposed to represent...
"...Many of the boons are extraordinary and represent the gradual transformation of a character into something resembling a demigod."
5e's boons also take the stance of allowing the particular table to flavor the fiction that surrounds these abilities as opposed to providing it and gives on just the mechanics behind each one. Which is consistent with how the system in general is presented.
D&D 4e's Epic Tier has been routinely derided by detractors as "Superheroes" because of these things above. Until now I suppose?
Well I've heard the entirety of 4e derided as a fantasy superhero game by those who take iussue with it... but that bears no relevance to this discussion which is whether 5e can emulate a mythic tier... which it can. the biggest differences between it and 4e are...
1. It is purely optional not a mandated part of the game (as 4e Epic Destinies were), even to the point that if one wants to play at level 20 and above with more grounded and less powerful/mythical alternatives to epic boons you can as they are also provided in the DMG.
2. The flavoring/exact fiction of the abilities, even whether they are magical or non-magical is not defined in 5e so the descriptions can be tailored to the feel of the particular campaign... or even the particular character.
Note that 5's level 17-20 Epic Tier advice cribs 4e's (1) exactly, though doesn't follow through as (a) there are no Epic Destinies to take heed of, (b) there are no actual superheroic abilities for martial characters that are non-magic (like the 4e utilities above) to guide content generation/genre tropes, and there is no attendant advice to look to the absent (a) and (b) for content generation. Further still, contrast the second sentence of (1) above with the design impetus (and related impacts on play) of bounded accuracy (which strives to keep low tier obstacles/threats relevant at endgame play).
a.) Epic destinies are not a requirement for mythic fantasy... Epic boons serve the same function.
b.) Nothing defines Epic boons as magical. They are specifically called out as "special powers". The flavor and designation is left open.
b. 1/2) The content generation is both addressed in the description of the tier as well as the section on mythic fanatsy in the DMG. I think that 5e avoids locking all this down at the level of specificity that 4e does (well except in adventures for some reason) because it is purposefully giving you the room to flavor to one's own tastes... as an example one person's Mythic may be Gilgamesh, Beowulf and Hercules while another groups may be Goku, Vegeta and Trunks. While the mechanical capability of these two groups would be similar the fiction would probably be colored totally differently
c.) The whole point of epic boons is to break bounded accuracy. 5e recognizes if you've chosen (and that is the really important distinction here that the group and DM must buy into it vs. it being integral to the game ) to play with Epic boons you are choosing to toss bounded accuracy out the window.