Apparently the Linux kernel can emulate MTE in software:
The software MTE support in the kernel apparently predates the hardware support of the same, based on the above link.
Admittedly I might be misunderstanding something here, my confidence level is only 65%.
p.s.: i just learned that this uses a lot of extra RAM compared to hardware MTE. there seem to be approaches to mitigate the RAM blow up, but this discussion is above my paygrade, so I have to bow out. That said, I think it is correct that the absense of hardware MTE should not be the end of the road for secure smartphone design.