what was the reason that the arity of function types maxed out at 22 originally? (before FunctionN). is there something special about 22 or was it arbitrary?

as far as I know, it's arbitrary, but curious. Scala also tops out at Function22/Tuple22/Product22, with a similar array-based type-erased workaround for larger arities as Kotlin's FunctionN

yeah similarities between scala and kotlin there is what made me wonder

It's arbitrary. Kotlin just picked the number from Scala. Why Scala tops at 22 I don't know.