MTE is not the only blocker here. Pixel 7 series do not have it and are currently supported by GrapheneOS. There are other concerns regarding things like a proper secure element implementation and timely firmware/binary blob updates.
Devices are supported until end-of-life rather than being dropped when they no longer meet the requirements. Pixel 6 is nearly end-of-life and Pixel 7 will be end-of-life in a bit over a year. Both have 5 years of updates from launch as opposed to 7 years for the Pixel 8 and later.
We want to require 7 years of updates for new devices rather than 5 but have left it at 5 to help budget devices meet our requirements.
Most people expect to have decent encryption without a strong passphrase, at least 5 years of security updates and a lot more. Fairphone says they provide updates far longer than they do for many components, and those come with substantial delays. Fairphone 5 and earlier have end-of-life kernels without security support. That's a very bad situation and is widely ignored. The more recent devices are headed to the same situation for the Linux kernel and other components.
Citation needed.