There is a useful Q&A with an authoritative source concerning individuals who held limited dual citizenship as children, are now over 21 years old, reside in Indonesia, and missed the 2022–2024 window for either electing Indonesian citizenship (WNI) or formally declaring their limited dual-citizenship status. However, it is unclear whether the outcome of this case can be generalised to others in a similar situation, as the individual was able to raise the matter directly with the minister and senior officials during this public forum.
Although he missed both windows of opportunities to regularise his status, the case appears to have been considered on the basis of fundamental principles of Indonesian law, namely that 'a person should not be rendered stateless (apatride)' and that the state should apply 'the principle of maximum protection'.
The only requirement appears he was asked was the proof that he did not hold any other citizenship. His father was able to obtain a statement from the Singaporean government confirming that he was not registered as a Singaporean citizen. In many other countries, unless for very compeling reason obtaining such a letter would be difficult or even impossible, as authorities may regard the disclosure of citizenship-related information as confidential or as an inappropriate interference in matters concerning another country's citizens.
As I understand it, he has lived in Indonesia for his entire life. In some cases, mistakes made by parents can have significant consequences for their children when the grow up, particularly when those issues only come to light years later.