Since this question was asked, Jeremy Crawford has changed his mind on how this works.
In the April Rules Answers column for Sage Advice, Crawford has this to say:
Does moonbeam [or Spirit Guardians] deal damage when you cast it? What about when its effect moves onto a creature? The answer to both questions is no.
He goes on to elaborate:
Reading the description of any of those spells, you might wonder whether a creature is considered to be entering the spell’s area of effect if the area is created on the creature’s space. And if the area of effect can be moved—as the beam of moonbeam can—does moving it into a creature’s space count as the creature entering the area? Our design intent for such spells is this: a creature enters the area of effect when the creature passes into it. Creating the area of effect on the creature or moving it onto the creature doesn’t count. If the creature is still in the area at the start of its turn, it is subjected to the area’s effect.
This is the list of spells the article applies to:
- blade barrier
- cloudkill
- cloud of daggers
- Evard’s black tentacles
- forbiddance
- moonbeam
- sleet storm
- spirit guardians
Contrast with his original tweet, and follow-up.
Master Chronology
Based on the link provided by Mark Horsfall, and the information Paul Westermeyer has compiled and compared in the linked article, the following information has been derived:
Sources
The three primary campaign settings for D&D are Greyhawk (GH), Forgotten Realms (FR), and Dragonlance (DL). These three have more material published than the other settings, and include the majority of the cross-time references. (Which other settings are: Spelljammer (SJ), Maztica (MZ, part of FR), Al-Qadim (AQ, Also part of FR), Mystara (MY), Ravenloft (RL), Planescape (PS), Birthright (BR), Athas (AT), Red Steel (RS), Odyssey (OD), Lankhmar (LM), Eberron (EB) and Conan (CO).)
There are four additional primary sources for chronological cross-time points: Spelljammer, Planescape, Ravenloft, and Dragon Magazine (DM) articles written by Ed Greenwood himself.
Please note that there are some contradictions in sources, so precedence is given to authors with more published works and to sources based on Spelljammer (as it is specifically designed to cross-bridge the settings) as well as the Big 3 (FR, GH, and DL).
FR/DL cross-time link
Based on comparative information from SJ, FR, DL, RL, and PS the best cross-time linkage is:
1361 DR <-> 358 AC
The RL data contradicts itself, weakening it as a source, and the PS data fails to trump the SJ data.
GR/FR cross-time link
Based on data from FR/RL/GH and DM sources the best cross-time linkage is:
1361 DR <-> 581 CY
Note that the DM articles were written by Ed Greenwood about the wizards Elminster, Mordenkainen, and Dalamar meeting in his home on Earth to discuss the magical affairs of Faerun, Oerth, and Ansalon with reference to various famous wars and other events; and as such provide a strong primary source for cross-time linkages. As such, the dates of these articles can be used to provide an Earth-time linkage as well, should such be desired.
Other cross-time links
Linkages to other settings frequently fall back on the publication date of the earliest release of said setting, combined with in-setting references, or lack thereof, to determine dates. For detailed reasonings, check the link if available - and I have saved a copy in case the link goes down.
Calendars
After all is compared, the following calendars can be synced, with confidence:
- Forgotten Realms: Dale Reckoning (DR), Shou Lung (SL)
- Greyhawk: Olven Calendar (OC), Common Year (CY)
- Dragon Lance: Ansalonian Calendar "Alt Cataclius" (AC)
- Spelljammer: Time of Unity (TU), Anno Vulkarus/Promo Novo (AV), Astromundi Chronos (AstroC)
5043 OC = 581 CY = 358 AC = 1361 DR = 2611 SL = 465 AV = 856 TU = 9/1299 AstroC
This corresponds roughly to the 1990's on our Earth, for those interested.
Best Answer
There is no reason to assume that the Spellplague impacted other realities.
For several reasons
Crystal Spheres aren't part of The Realms campaign setting, they are added by Spelljammer.
First and foremost, understand that the concept of Crystal Spheres, and every other setting existing in a different one floating out in The Phlogiston is NOT core material to The Realms. It was added for the Spelljammer Campaign Setting as an optional way to tie all of the different settings together. In the FR wiki article linked in the question, note that the only source cited is the Concordance of Arcane Space...which is a Spelljammer Rulebook. The Planescape Campaign Setting offers an entirely different optional way to tie the settings together. This was explicitly done to let you play in multiple different settings with the same characters. It is, essentially, a hack applied on top of the Core settings.
The core settings (Forgotten Realms, Dragonlance, Greyhawk, Eberron, etc) all operate under the assumption that other settings either don't exist, or are sufficiently disconnected as to not matter. You never see a setting book explaining how this thing that happened in Eberron impacted these other things that happened in Faerun. They simply assume that the setting you are playing in is the only setting that matters.
Thus, outside of these two optional settings, travel between the two is impossible and Oerth and Faerun may not even exist relative to one another. (In a game played in Faerun, Oerth and 'Greyspace' may not exist at all). Based on that alone, each individual setting can be considered to be entirely cut off from one another, and cannot impact each other. Because without one of two optional campaign settings, no connection between them exists.
Nature of the Spellplague
The Spellplague was the result of a breakdown in the Weave of Magic caused by the murder of Mystra, goddess of magic. With no deity to manage the weave, it went rampant and caused a huge mess.
Thing is...
In Greyhawk, Boccob is the god of magic. In Krynn, it's Lunitari, Nuitari, and Solinari. In Eberron, there is no god of magic, magic is (theoretically) derived from Siberys, the golden ring around the planet (and dead progenitor dragon).
And, according to the Spelljammer rulebooks (concordance of Arcane Space p17-18) deities have no impact whatsoever on any crystal sphere they do not hold sway in. Unless they have a pool of worshipers there, or a powerful cleric who can make a connection, they can't interact with that sphere at all. The death of a god recognized in only one Sphere cannot impact another sphere.
Differing cosmologies
The settings in D&D are extremely different. In terms of settings that go into significant detail on cosmology...The 'World Tree' cosmology was something unique to the Faerun setting.
Greyhawk was left intentionally vague in terms of cosmology. Krynn Cosmology looks like this. Eberron Cosmology looks like this. Old Faerun Cosmology (World Tree) looked like this, and the new version like this. See how different they are? There's no way these are the same planes in the same Astral Sea.
In Conclusion
The simplest answer is that, according to the core settings, nothing that happens in one setting impacts anything that happens in another setting, because they are different settings. The fact that two additional settings (Spelljammer and Planescape) were invented to let you optionally glue them all together doesn't change the fact that they are entirely discrete settings with their own history and events that do not impact each other.