South Korea is quite different from Taiwan in this regard. Before Taiwan’s democratization, the anti-communist laws were repealed by the Democratic Progressive Party (DPP) after democratization. This was because, during the White Terror era, many predecessors of the DPP were accused of being "spies" by the Kuomintang (KMT) and were executed. The anti-communist laws were closely associated with the KMT's White Terror, so after democratization, Taiwan directly abolished laws like the "Act for the Control and Punishment of Espionage," the "Taiwan Martial Law," and the "Act for the Control and Punishment of Rebellion."
However, in South Korea, although some adjustments were made to the sentencing of the National Security Act after democratization, the law itself was not repealed due to the significant influence of conservative forces, and it is still supported by the Constitution. In Taiwan, it is theoretically legal to form a communist party, openly wave the Chinese flag, or advocate for unification. In fact, the Communist Party even has seats in the Hsinchu City Council. This is why you can see elderly members of the Unification Promotion Party waving Chinese flags and advocating for unification on platforms like Bilibili.
South Korea, on the other hand, is different. Due to the existence of the National Security Act, if you publicly advocate for North Korea to unify South Korea, although you won’t be executed as during the authoritarian regimes of Park Chung-hee and Chun Doo-hwan, you could still be arrested. This law applies not only to South Koreans but also to foreigners. For example, a Chinese student studying in South Korea was deported for publicly mourning the death of North Korea’s second leader and supporting North Korean unification of the Korean Peninsula.
The fundamental reason lies in the fact that South Korea’s White Terror was enforced through military government authoritarian rule. While the military government was anti-communist, it didn’t represent the entirety of the conservative faction. As a result, even after democratization, the anti-communist conservative forces remained strong. In Taiwan, however, the White Terror was enforced through the centralized control of the KMT. Under the wave of democratization, the KMT was forced to transform and give up its original anti-communist ideology. In fact, it even began cooperating with the Chinese Communist Party (CCP) on issues like cross-strait unification, which meant it had no stance to maintain the original anti-communist laws.
Ironically, the forces that pushed for Taiwan's democratization—due to their stance on the unification-independence issue—actually had more motivation to enact anti-communist laws. However, because of their priority to oppose the KMT, the independence faction could not support the executions of "spies" during the White Terror, even establishing the Transitional Justice Commission to rehabilitate those accused as spies during that era, including individuals recognized as martyrs by mainland China.