That is completely untrue. The ROC constitution only mentions Taiwan once and it's in the newer additional articles. You have the phrase "free area of the Republic of China" which is a rather explicit claim that the ROC still formally claims the entirety of China, which includes the Mainland as the "not free" part of China. In summary, there's the ROC in its entirety, there is the free area of the ROC (Taiwan, Kinmen, Penghu), and there's the not free area of the ROC (Mainland, Hong Kong, Macau).

But we know they had not done so is because if they had actually dropped claims to the Mainland, the PLA would've long since invaded Taiwan and overthrow the ROC. To rescind claims to the Mainland is to de facto uphold Two Chinas, which is considered a red line by the PRC on par with Taiwanese separatism.

If you're talking about actively waging war to "liberate" the Mainland from the CPC, active war stopped by the time of Chiang Ching-kuo when the ROC was still under martial law. Liberalization under Lee Teng-hui didn't change the material reality that the ROC was no way militarily capable of overthrowing the PRC and reestablishing ROC control of the entirety of China.