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).
Aha! Yeah the last time I needed to play with timezones I had map between Olson and Windows's proprietary names. Looks like they came out their NIH Syndrome:
"Proposed" is the final state of many stable RFCs; see a list at [1], and compare it to the (much shorter) list of finished standards above it. The name is kind of misleading these days. Some discussion at [2]. Just to give an example, the base64 RFC 4648 is also a "proposed standard".
