In the year or two before the release of the new PHB, WotC made quite substantial changes to the way the D&D business operates, making it very difficult to apply what we know about earlier eras to figure out what's happening now.
WotC now sells books direct-to-consumer and incentivizes purchase by deeply discounting physical/digital bundles. How successful is that program? How many sales has it pulled away from Amazon, FLGSes and other outlets? We have no idea.
WotC ended its relationship with Penguin-Random House and now sells to Amazon directly. (A hefty percentage of D&D is sold through Amazon.) An important consequence of this change is that Amazon now treats D&D products more like games, while they were previously treated like books. This changed a number of things, including the way Amazon discounts the products. During the PRH era, Amazon customers almost never paid cover price even for the very latest D&D products. These days, it seems, new products are barely discounted at all which is certainly impacting sales. WotC is pleased about this. Their aim is to take as much of the business direct-to-consumer as possible; they want to engineer a world in which WotC itself is the only source of discounted D&D (at least on the frontlist). In effect, WotC is deliberately selling fewer books in an effort to earn more revenue on each of those sales. Is that working? We have no idea.
WotC launched a VTT and are using it to drive DDB subscribers? How is this impacting Roll20? Is it incentivizing more digital sales? We have no idea.
The OGL fiasco blew up the release schedule for 2023-24, the 50th Anniversary was poorly exploited, and the roll out of the 2024 core books was suboptimal. All of those factors had at least a short term impact on the business. How significant? We have no idea.
It shouldn't surprise anyone that more customers purchased the 2014 PHB on Roll20 over a ten year span than purchased the new PHB in four and a half months. Similarly, the DDB data posted upthread looks pretty dubious.
Is D&D experiencing a "slow down?" I've yet to see a compelling argument and I can see several counter-arguments. Still, we don't know. It's not at all unlikely that growth is slowing down--you can't continue to grow at 20+% annually forever. There's no question the brand remains at or near its high watermark.