The Multiverse covers everything
D&D has a conceit that all campaign settings are embedded within an “oversetting,” known (currently) simply as “the Multiverse.” There are a large number of things that more-or-less “multiversal,” that is, they behave the same way everywhere in the multiverse—within each campaign setting as well as outside any of them, on the other planes.
Thus, it makes perfect sense to ask a question about lore within D&D that isn’t specific to any one setting, because there are things generally shared by all settings. This is, in fact, Wizards of the Coast’s preference for nearly everything—they like to keep D&D pretty consistent across campaign settings when they can, because it makes it easier for players to get into new settings (read: become convinced to buy new products). Thus, most stuff—magic, spells, classes, feats, etc. etc.—all have the same lore behind them no matter where you go.
As for the “default” of the Forgotten Realms, Wizards of the Coast has been adamant that this is not the case for 5e. They have exported a number of things (e.g. the Weave) that were previously specific to FR, making them general concepts throughout the Multiverse, but officially, that isn’t because FR is the default, it’s because they’re ret-conning every place else to be a little more like FR. (And FR has had some of this coming back at it, particularly the necessity of patron deities.)
The Multiverse is also its own setting
The Multiverse is also setting in its own right, with its own canon, history, characters, and so on. It is entirely reasonable to ask questions about it. Though it contains the Forgotten Realms, and Greyhawk and Eberron and Krynn and so on, plenty of things go on outside of those, and often go unnoticed within those—vagaries of the Blood War across the Lower Planes rarely influence life on the ground in Toril, for example.
It might be reasonable for there to be a dnd-multiverse for questions about these things, but I don’t see a lot of need for it, personally. I wouldn’t apply that to questions about, say, how spellcasting works within D&D’s lore—even though that is “multiversal,” it isn’t specific to the Multiverse as a setting, it applies to pretty much all D&D.