Well, answering to myself (and in case anybody else is wondering the same by now): no, it's not mandatory to use 3.2.2 (or above at the moment of writing this) as my game still uses 3.2.1 (even if 3.2.3 is already stable now), and there has been no problem with Android Billing.
Also, about iOS, as it's not the same singleton, there's no problem in that platform regarding the Godot version in use (puls 3.2.1 is quite stable in that regard)