> Adding a warning sounds like a good idea.  Is it reasonable to include
> a recommended cross-platform approach in the platform doc, like either
> the sys.maxsize test or the struct.calsize("P") test (which is used as
> a default fallback in platform.architecture)?

Yes.

>   Are there any currently supported platforms where either of those
> wouldn't work?

No. I think even on fringe platforms it is unlikely for size_t to not
reflect the native pointer width (although it is theoretically
possible).