Ah, thank you.

Now that I'm back at my desk I had to check ISO8601, that suffix is not included. However it does look like an extension - RFC 9557 - which looks like is still in proposed state.

I would personally caution using these suffixes until wider adoption, because AFAIK the Olson database names themselves are not standardised on non-POSIX systems (i.e you might have a hard time on Windows).