As mentioned in another reply, there are already a couple of tagging approaches in production.

Unfortunely x86/x64 don't have any, and MPX was broken from the get go.

This isn't a memory tagging system at all and has capabilities far beyond that (pun intended), so I don't know why whatever other approaches like MTE are out in the wild are relevant.

They're relevant because they're technologies relating to memory safety and provide some level of additional protection. However, they rely on secrets and are in general only probabilistic, so they don't deterministically mitigate all memory safety issues (you can deterministically mitigate some with clever allocations of memory "colours", but not all). CHERI and MTE-like schemes also both rely on the use of tagged memory, but in rather different ways.

They are 100% relevant in what concerns taming the native code produced by C derived languages.