The Singapore passport's first page bears the following statement:
This passport is valid for all countries except the following:
What is the purpose of this page? Does Singapore reserve the right to ban its citizens from travelling to specific countries?
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.)
I find it very hard to find clear information on this, but as far as I can tell, this page is where limitations to the passport would be posted by the state that issues the passport (in this case Singapore). There are general limitation some countries issue on their citizens, banning them from travelling to certain other countries. Reasons might be safety or might be of political or religious nature. Singapore does not have such restrictions, hence the page is generally empty.
For illustration, here are some passports from other countries with similar entries:
What is the purpose of this page? Does Singapore reserve the right to ban its citizens from travelling to specific countries?
That's exactly what it is. While I believe that the list mentioned above (China, North Korea, Cambodia, Laos, Viet Nam) is empty today, cousin Malaysia still makes its passport invalid for Israel.
Singapore could reinstate travel bans to X, Y, Z, although I don't see it happening. They're difficult to enforce – especially if these places issue a piece of paper instead of stamping the passport – and don't mean much today.
Something I forgot to mention: remember that in most (all?) countries, your passport is not really "yours": it belongs to the government, which can decide at any time to withdraw it, not renew it, etc. Obviously some countries take that a little more seriously than others.
