They managed to avoid talking about Planeswalkers in Guildmaster's Guide to Ravnica, but I think that will be harder in Mythic Odysseys of Theros.
I mean how do you explain where Thassa's Bident went, or how Heliod got stuck in the Underworld, or the now dead God of Revels, Ashiok like at all.
Actually speaking of Thassa's Bident, how are you supposed to get it back when it's on Zendikar?
Anyways if Zendikar gets a book in 2021 as I and others strongly suspect, it's almost impossible to avoid talking about the Eldrazi, they've shaped too much of it's history. Its what caused the roil, one of them lead to the Vampires, they were worshipped as Gods for a time, sort of, and sooooo much more. Oh and they nearly destroyed all other creatures on the world.
I mean Zendilar rising is going to have to explain how they put stuff back together again including healing the land.
And it will even be hard to avoid talking about Planeswalkers, who played such a huge role in the story of Zendikar.
Doing Theros without explaining planeswalkers isn't that hard: just describe events as the people of Theros at large believe them to have happened. Given how the blurring of myth and fact is one of Theros' big
things, there's plenty of room for vagueness and uncertainty about the nitty-gritty details.
"A very clever triton tricked Thassa and escaped with her bident."
"A new god with the visage of a satyr emerged, but Heliod's champion slew him before Heliod betrayed her, fearful of her power, killing her with the very weapon he had given her."
Ashiok worked in the shadows, so they wouldn't need to be covered in either notable past events or notable individuals that could be encountered, though "a mysterious mage with an unsettling visage performed arcane experiments into harvesting magic from people's dreams" isn't something that requires mention of planeswalking.
The events of Beyond Death probably won't come up if the Theros book handles things the way GGTR did, setting itself slightly before the most recent events, but even then, "the former Sun's Champion escaped the Underworld, confronting and defeating Heliod when he emerged to stop her, and jealous Erebos took the opportunity to trap Heliod in his domain while the Champion left for distant lands, pursued by an Agent of Fate" should work fine?
Zendikar is harder to do without ever saying the word "planeswalker," though not impossible: the Nahiri/Sorin/Ugin triad can be described fine as "ancient immortal mages" (not exactly a foreign concept to D&D), and the Gatewatch are straight-up just a team of traveling adventurers who showed up, accidentally released a sealed horror, helped defeat said sealed horror, then continued on to their next adventure - the fact that they happen to be able to walk between worlds (and, thus, the question of anyone else
not being able to do so) doesn't have to come up.
It's outright impossible to do Zendikar without saying the word "Eldrazi," but given that they're specifically based on a genre wherein the monsters' Deal is intentionally
unknowable, I feel like not getting into the details of how they work from a cosmological standpoint would be pretty reasonable? They're ancient, unfathomable aberrations that were magically sealed within the world, their very presence warping said world until they were released, almost destroyed the world, and were destroyed themselves, though a few of their spawn may still roam the devastated, but recovering, land. No mention of the Blind Eternities necessary.
TL;DR: I think you might be overestimating how difficult it would be to continue avoiding this question, given that the details of planeswalking, and the details of the Eldrazi, largely aren't directly relevant to telling stories based on individual planes. A concrete explanation would be
nice, sure, but leaving the details for the DM to work out on based on their personal preferences
* seems entirely on-brand for 5E.
* More D&D-focused DMs can just use D&D cosmology and not care that it's not perfectly MtG canon, DMs with more basis in MtG can set more stringent homebrew rules about interplanar stuff that match with MtG's cosmology.
) Aberrations.
