Plato's forms, if followed sufficiently, would be 1-A (At the very least), since they exist in a higher reality as perfect, eternal, and unchanging archetypes beyond space and time that our (lower) reality is just an imitation of.
A series just having platonic concepts/forms doesn't make it 1-A though. Requires further context and still must abide by tiering standards.
The Form of Good on the other hand would be Tier 0 if properly depicted, since it illuminates all other forms (qualities) in the world. Essentially acting as the ultimate reason behind everything.
Conventional Set Theory ends at Low 1-A.
I would recommend reading the tiering system pages that explain it.
1-A is basically when something exceeds the compositional makeup of the prior tiers. Something that can't be reached by stacking any degree of lower existence upon itself.
So like you can multiply spatial objects infinitely to reach into higher and higher dimensions, that sorta thing would never allow you to reach a 1-A being since they are utterly beyond spatial composition. To the point that it might as well be fictitious/nonexistent to them.