I think this is out of scope for ISO 8601, especially since the "Europe/Bratislava" label is a specific reference to the tzdata tables. Another system may use another way of specifying the location for the time zone, e.g. GPS coordinates. (Though in practice I don't think anyone does that; literally everyone uses tzdata.)

The "correct" way is presumably to work with an 8601 timestamp without time zone and carry the time zone spec along in an additional field.