Answer
Is there a single official list of countries eligible for APC waivers?
No. There is no common list. Publishers derive their own from Research4Life groups, World Bank income bands, EIFL agreements or their own criteria, then add further conditions such as GDP ceilings. Two publishers citing the same framework still publish different lists, so eligibility has to be checked per publisher.
Detail
Where the published lists are
Of the fifteen publishers checked, only Wiley and Taylor & Francis publish complete country lists. The rest state a rule and leave the list to be inferred, or hold it behind an eligibility checker.