If x16|x8/x8+x8+x4 it was possible I’d expect it to have happened by now. Closest I know of is x16+x4+x4, which is rare in AM5 but looking common with Z890 chipset x4s.
FWIW the Granite Ridge IO die seems to have an x16 PHY and three x4 PHYs, consistent with X870E emphasis on PEG bifurcation, but if there’s any explanation why two of the x4s can’t run x8 I haven’t been able to find it.