The early ARM Cortex MTE support has full support for synchronous and asymmetric (synchronous on reads, asynchronous on write) modes. Asynchronous was near zero cost and asymmetric comparable to a mitigation like MTE. This has been available since the launch of the Pixel 8 for Android. GrapheneOS began using it in the month the Pixel 8 launched after integrating it into hardened_maloc. It currently uses mode synchronous for the kernel and asymmetric for userspace. EMTE refers to FEAT_MTE4 which is a standard ARM extension with the 4th round of MTE features. It isn't Apple specific.

MTE is 4 bits with 16 byte granularity. There's usually at least 1 tag reserved so there are 15 random tags. It's possible to dynamically exclude tags to have extra deterministic guarantees. GrapheneOS excludes the previous random tag and adjacent random tags so there are 3 dynamically excluded tags which were themselves random.

Linux kernel MTE integration for internal usage is not very security focused and has to be replaced with a security-focused implementation integrated with pKVM at some point. Google's recently launched Advanced Protection feature currently doesn't use kernel MTE.