Kerfluffle Is there anybody here who knows the answer to this? Any developers who would know?
I think there is likely to be only accidental overlap between GrapheneOS developers and people who could truly answer this question.
First I think it's important to point out that the "automatically broadcasts its IMEI as part of its hardwired initialization process" bit from the other thread is pure speculation which does not cite any sort of evidence. It could be happening, but other kinds of leakage could be happening too! In theory the Wi-Fi hardware could sneakily leak a couple bits of the true hardware MAC each time it associates with an access point. I just made that up right now, but it could be happening!
It might be productive for somebody with cellular radio expertise and equipment (e.g., a fake cell site, "Stingray", etc.) to report on what cellular traffic, if any, is observed as a Pixel with GrapheneOS set to "airplane mode" boots. But it is at least fairly plausible that none of the GrapheneOS developers have that kind of equipment lying around.
The question is interesting, but asking a friendly local cell-tower technician to look into it might make more sense than asking the GrapheneOS developers. Please note that I don't speak for the GrapheneOS project.