In the past, for fear of communism, the Singapore government banned Singapore citizens below a certain age (I think 40 but I'm not sure) from visiting the PRC, North Korea, Cambodia, Laos, and Viet Nam.
From a Reddit post (passport said to be from the "early 80s"):
As you can see, the above passport page was amended to remove the ROC (Taiwan).
Later (I'm guessing c. 1980s), it was further amended to also remove the PRC, North Korea, Cambodia, Laos, and Viet Nam.
Since 2017 the front passport pages look like this:
My guess is that the government of Singapore chose to retain this page so that it would have the option of banning specific citizens from visiting specific countries, though I'm not sure this option has ever been used. (Political opponents and those with terrorist sympathies have gotten travel bans, but these are usually complete bans on leaving Singapore at all, rather than bans on visiting particular countries.)