As far as I know, they have no application apart from academic toy/reseearch subject right now. And you have to consider that there are a lot of niches for storage technology that they could have taken over (because there is a lot of tradeoffs to make, e.g. latency, bandwidth, persistence, density, power consumption).
We might be just a few breakthoughs from those things replacing flash memory in SSDs, or revolutionizing neural-network accelerator hardware, but I am quite skeptical for now.
Note: I still believe that this (and other stuff i'm skeptical about) is SUPER worthwhile to research and always a huge uphill battle, simply because we have invested hundreds of billions of dollars into improvements of CMOS technology and processes, and collected over half a century of experience with it...
But new tech is to me kinda like a startup-- not every technology is the future, just like not every startup is a unicorn. Investing is still the right move, but you have to be realistic about expectations (which modern media is absolutely not)
There is still, to this day, a numerical niche for these drives, which is being served imperfectly by either normal TLC drives of very large size, SLC cache drives, or DRAM expansion cards connecting to the CPU through a PCIE bus. Just not at the prices they wanted to charge.
I recall their early R/W speed performance projections being far faster than what they ever achieved with Optane drives.
WP:
"Development of 3D XPoint began around 2012.[8] Intel and Micron had developed other non-volatile phase-change memory (PCM) technologies previously;[note 1] Mark Durcan of Micron said 3D XPoint architecture differs from previous offerings of PCM, and uses chalcogenide materials for both selector and storage parts of the memory cell that are faster and more stable than traditional PCM materials like GST.[10] But today, it is thought of as a subset of ReRAM.[11] According to patents a variety of materials can be used as the chalcogenide material.[12][13][14]
3D XPoint has been stated to use electrical resistance and to be bit addressable.[15] Similarities to the resistive random-access memory under development by Crossbar Inc. have been noted, but 3D XPoint uses different storage physics.[8] Specifically, transistors are replaced by threshold switches as selectors in the memory cells.[16] 3D XPoint developers indicate that it is based on changes in resistance of the bulk material.[2] Intel CEO Brian Krzanich responded to ongoing questions on the XPoint material that the switching was based on "bulk material properties".[3] Intel has stated that 3D XPoint does not use a phase-change or memristor technology,[17] although this is disputed by independent reviewers.[18]
According to reverse engineering firm TechInsights, 3D XPoint uses germanium-antimony-tellurium (GST) with low silicon content as the data storage material which is accessed by ovonic threshold switches (OTSes)[19][20] made of ternary phased selenium-germanium-silicon with arsenic doping.[21][22]"
I mean, that's not because graphene has become a routine part of our material repertoire. It has no reason to be in those things, does nothing, and is just marketing fuel. We may put "graphene" in things, but we are not much closer to using its interesting properties.
We don't put it on a lot of things. It's expensive as hell.