Trying to contact an anonymous reviewer might be considered unethical because it is an attempt to deanonymise someone who has provided a report under the assumption they are anonymous. While this is especially true if the report is critical, it remains theoretically the case irrespective of whether the report is positive or negative.

There may be particular special circumstances where breaking that anonymity may be appropriate, usually with the reviewers permission.

This obviously doesn't apply to a reviewer that identifies themselves. Although I'd assume that they had given all the feedback they had to give in their review.

Trying to contact an anonymous reviewer might be considered unethical because it is an attempt to deanonymise someone who has provided a report under the assumption they are anonymous. While this is especially true if the report is critical, it remains theoretically the case irrespective of whether the report is positive or negative.

There may be particular special circumstances where breaking that anonymity may be appropriate, usually with the reviewer's permission.

This obviously doesn't apply to a reviewer that identifies themselves. Although I'd assume that they had given all the feedback they had to give in their review.