eOS uses microG. I'd wish Fairphone offered partnership with GrapheneOS, especially now that Google broke their workflow. Sandboxed Play Services is pretty much a must for a lot of people.
The makers of GrapheneOS have indicated that Fairphone doesn't meet their security requirements:
https://grapheneos.social/@GrapheneOS/114737139118874189
I think there are some fundamental flaws with how Fairphone operates, plus they don't seem to release security updates promptly.
Case in point: re-lockable bootloader requirement. Not everyone is a target for an evil maid types of physical attacks or possible state actor pressure. But when you actually need it, it's not negotiable.
GrapheneOS takes security very seriously. Your average desktop PC or laptop won't come close to their requirements. That makes GrapheneOS an excellent OS for people who want the security of iOS without the many downsides of Apple. Their patches reduce usability but make the phone more secure than Google's own, official Android build.
However, if you've ever used a Windows (or Linux) laptop, you've already experienced the kind of insecurity that GrapheneOS tries to prevent. No hardware encryption accelerators outside of the CPU, rarely any patches that roll out within a weak of announcement, firmware protection being basically nonexistent, no A/B updates, almost certainly no verified boot (even with Secure Boot enabled), and usually no firmware USB lockdown.
They did announce they're going to do daily linux patches though, so that's atleast something https://www.phoronix.com/news/Fairphone-6-Linux
It would be good if Fairphone could make a product that meets GrapheneOS requirements, but they measure the tradeoffs between security, usability, and cost (to do hardware and software things) differently. Each team is free to make the choices they deem fit. If only the intersection of GrapheneOS and Fairphone users were bigger, market forces would push them towards a common vision.
There's is no bad blood here, it's merely that fairphone doesn't meet the required standards for them to be a target the graphene team is interested in supporting offically. There's nothing preventing anyone from porting it themselves and nothing preventing fairphone from porting an inferior version of grapheneos to their phoens.