No, there is not
It is always hard to prove a negative, but I am fairly confident that no such explanations exist. There are some in-world modules to justify changes associated with edition transitions in general (Fate of Istus and Die, Vecna, Die!), but neither of them does address psionics, or even more specifically, monster psionics, explicitly.
White Box to 1e transition. In the white box edition of D&D "outsiders had psionics" only when using supplements. The entire text of the white box does not contain either of the terms "outsider" or "psionics" a single time. Psionics were originally introduced in the Eldritch Wizardry supplement to the white box, and then added in slightly altered form to the 1st Edition of AD&D, which was mostly a gathering up of the original white box and supplemental material since its publication, and a few added rules either from Gary's home campaign or lobbied for by others, with no major changes. Because of this continuity, there was no need to explain any changes.
Greyhawk was not the "default" setting during the White Box/1e/2e era, there were multiple settings all used in parallel, and the rules were meant to be generic, even if they included spells named after personalities from Gary Gygax' home Greyhawk campaign, like Tenser, Mordenkainen and Otto.
During the 1e/2e transition, for Greyhawk, Fate of Istus was published to justify the changes in game mechanics for the various classes. The book consists largely of the description of 10 cities with associated adventures that test the various 1e classes, and is not really talking in detail about any mechanics changes, other than mentioning the abilities of various classes change, and that some classes, for example Monks (which were not a core class in 2nd edition) are not available any more:
Those who remain in the Flaeness will find their skills fading: they may become either fighters or thieves with the same number of hit points.
So the entire module was only concerned about how to narratively explain the changes to exisiting PCs and their classes, not about explaining changes to monster abilities.
During the 2e/3rd Edition, Greyhawk was made the default setting, and there was the Die, Vecna, Die! high level adventure to explain the changes in cosmology between 2e and 3e, like the vanishing of the Demiplane of Ravenloft or Spelljammer from the officially supported material, the changes summarized as follows (p. 151)
Some Outer Planes drift off and are forever lost, others collide and merge, while at least one Inner Plane runs ”aground” on a distant world of the Prime. Moreover, the very nature of the Prime Material Plane itself is altered. Half-worlds [...] multiply a millionfold, taking on parallel realism in what was before a unified Prime Material Plane. The concept of alternate dimensions rears its metaphorical head, but doesn’t yet solidify, and perhaps it never will. New realms, both near and far, are revealed, and realms never previously imagined make themselves known. Entities long thought lost emerge once more, while other creatures, both great and small, are inexplicably eradicated. Some common spells begin to work differently
There is nothing specifically on psionics. For 3e, there was (to my knowledge) no introductory kick-off adventure to transition existing characters into it, or no other in game explanation for the change. The Sunless Citadel was the intro adventure, but it served to intoduce new characters at level one to the game, not to transition old hands with high level characters over. For that, WotC published a Conversion Manual. The conversion manual however was focused entirely on how to convert/translate characters statistics, and was not an in-world explanation.
4th Edition did use the "points of light" default campaign style and setting, so again, no in-game explanation for Greyhawk.
5th Edition finally adopted the Forgotten Realms as the default setting, so no in-game transition explanation for Greyhawk either (which was relegated mostly to a list of deities at the back of the PHB, and has no dedicated published adventures). In fact, several originally Greyhawk characters and adventures have been retconned or made generic, for example, Iggwylf, originally an evil archmage in Greyhawk, or in Tales from the Yawning Portal the above-mentioned Sunless Citadel, Against the Giants, and Tomb of Horrors all are given instructions of how they can be placed in several settings: Dragonlance, Eberron, the Forgotten Realms or Greyhawk.
Add to that that the psionic rules were always a bit of an ill-fitting stepchild (Gygax regretted ever including them in the 1e rules), and they were not even part of the core rules after 1st edition, and it seems pretty likely that no such official material exists.
P.S. The term Outsiders in 3.5e normally refers to any kind of creature not native to the prime material plane, in particular, genies demons, devils, and celestials; while you seem to refer to lesser deities or demigods when using the term. At least within 3.5e context, that is not the common use of the term.