Since no one has mentioned this yet:

C23 already has [[reproducible]] and [[unsequenced]], which are
designed exactly for this purpose. Here is an overview:
https://stackoverflow.com/q/76847905/5740428

There's a reasonable argument that we should have these in C++ for
compatibility and optimization. I wouldn't want to see any novel
design such as [[pure]].

Also note that the C23 attributes closely match [[gnu::pure]] and
[[gnu::const]], although there are some minor differences.