I am not sure even de jure the situation is clear. I think the following is the case (but I'm not a legal scholar, so please correct me if I'm wrong!)
When Japan lost the war, Taiwan was governed by the Allied Powers, who appointed Chen Yi from the Kuomintang (KMT) as chief executive and garrison commander of what was called "Taiwan Province". The Peace Treaty with Japan (Treaty of San Francisco) and the Sino-Japanese Peace Treaty (Treaty of Taipei) both did not specify that Taiwan was a part of mainland China. I think formally/legally Taiwan was Japanese until 1952 but Japan was under allied control at the time. The question of Taiwan's legal status was left open in the aforementioned peace treaties. This is the legal/technical root cause for the problems with Taiwan's legal status even today.
So, as far as I can see: de facto Taiwan is an independent nation. And de jure it's ... unclear.
Finally, let's bear in mind that in a very real sense there is no such thing as "international law", much of what is called international law is really just voluntary consent of states.
When Japan lost the war, Taiwan was governed by the Allied Powers, who appointed Chen Yi from the Kuomintang (KMT) as chief executive and garrison commander of what was called "Taiwan Province". The Peace Treaty with Japan (Treaty of San Francisco) and the Sino-Japanese Peace Treaty (Treaty of Taipei) both did not specify that Taiwan was a part of mainland China. I think formally/legally Taiwan was Japanese until 1952 but Japan was under allied control at the time. The question of Taiwan's legal status was left open in the aforementioned peace treaties. This is the legal/technical root cause for the problems with Taiwan's legal status even today.
So, as far as I can see: de facto Taiwan is an independent nation. And de jure it's ... unclear.
Finally, let's bear in mind that in a very real sense there is no such thing as "international law", much of what is called international law is really just voluntary consent of states.