It is (to my mind) blisteringly obvious that the Realms, and the Sword Coast region in particular, was, at 5e's outset, as close as it gets to being a "default" setting for D&D,
de facto if not formally designated as such.
Adventures
The vast majority of adventures published to date are set in the Realms, with one anthology that included non-Realms adventures,
Tales from the Yawning Portal, linking them to the Realms by means of the eponymous locale.
It's only since the pandemic that we've hit an inflection point where more adventures aren't set in the Realms than are.
Setting Books
The first setting book, published less than a year after the DMG, was the Sword Coast Adventurer's Guide, and it would be several years before another is published in book form (Guildmaster's Guide to Ravnica)
Supplements
Both Volo's Guide to Monsters and Xanathar's Guide to Everything are thematically tied to the Realms because their titular characters are from there. The tie-in is superficial, though, but not on-existent.
Even so, as has been noted upthread, it's still quite different compared to Nentir Vale in 4e or Golarion in Pathfinder, which are
unambiguously default settings.
I would say that 2018 is the year that things start to change:
- Mordenkainen's Tome of Foes (which links to a Greyhawk character) is published in May
- Ravnica is published in November
- Ghosts of Saltmarsh is published in 2019, and non-FR adventures become more common as noted above.
Since 2018, it's also clear (to my mind) that FR is no longer the
de facto "default" setting of the game. That position is more fully taken up by the D&D Multiverse, as has been clear in statements from designers, remarks in playtest documents, and the published materials:
Adventures
As noted above, since the pandemic, more often than not adventures are
not Realms-focused, although the Realms are hardly being neglected.
Setting Books
I don't know anything about Acquisitions Incorporated, so I'll leave that aside for others to clarify, but none of the other setting books published since SCAG are for the Realms. We've also now had two slipcase box sets for Spelljammer and Planescape.
Supplements
Starting from Mordenkainen's Tome of Foes, the supplements are no longer thematically tied to the Realms - indeed, apart from Fizban they're more Greyhawk themed, what with Tasha's Cauldron of Everything, the Mordenkainen monster remix, and Bigby's book on giants. (That said, just as with the Volo's Guide and Xanathar's guide, the theming isn't that strong.)
Whether this change has been for the good for the Realms, I couldn't say.
Considering the movie and BG3 are both set in the Realms, it might be premature to truly say the Realms is no longer the
de facto default setting of D&D, although I suppose that depends on whether one is looking only at the TTRPG products or not - if you restrict your analysis to TTRPG products, then the above analysis hold good (so I would assert).