Time zone designators can only be represented as value offsets from UTC, which means that it can't refer to an actual location. Time zones change for political reasons all the time, so using ISO8601 for future events becomes buggy. If Germany changes their policy on Daylight Savings, ISO8601 times such as 2021-08-02T09:00:00+0200 will have a 50% chance of suddenly becoming wrong (because they no longer refer to Berlin time).

Another problem with ISO8601 is that omitting the time zone info implies floating time rather than UTC time, and this causes no end of bugs in all implementations.

When I finally gave up on it, I came up with the following alternative:

Time zone data omitted (implies UTC):

2019-01-23/14:08:51.941245 = January 23, 2019, at 14:08:51 and 941245 microseconds, UTC

Area/location based timezone:

1985-10-26/01:20:01.105/America/Los_Angeles = October 26, 1985, at 1:20:01 and 105 milliseconds, Los Angeles time

Latitude/longitude based timezone:

5192-11-01/03:00:00/48.86/2.36 = November 1st, 5192, at 3:00:00, at whatever is in the place of Paris at that time