I got this year's signature and am pretty much pleased with it. I'd like to have GrapheneOS for it, too. But I guess, hardware (or marketing) is incompatible.
Signature 2026 can't be supported since it used the Snapdragon 8 Gen 5 which had defects in the MTE implementation and couldn't support it. The defects were resolved for the Snapdragon 8 Elite Gen 5 but Motorola didn't end up having any device with it. Signature 27 is a much higher device than the Signature 2026 and moved up to a higher tier above the Elite SoC. There will be cheaper Motorola devices supporting GrapheneOS in the future but this is the first providing everything it needs.