Yes, they are with one GPU core fused off. I came across a die shot on the internet[0], and the GPU cores look huge. With some rough calculation, I estimated that the GPU cores together take up about 15.5% of the die area.
I don't know much about photolithography, but I assume the same percentage of single-defect dies could be limited to a single GPU core failure, it's actually pretty surprising that Apple can get enough of these "rare draw" chips to build and ship a real product.
If the shortage continues, I would expect that they start using fully functional A18 Pro chips with one GPU core disabled with software. It kind of reminds me of the AMD Athlon days when user could use a pencil to unlock extra cores.
[0]: https://chipwise.tech/our-portfolio/apple-a18-a18-pro-die-sh...
