Is there a resource/database somewhere that has a list of what FME release each version of a transformer was initially released/updated in?

You can get it pretty easily for transformers on the Hub, but not for inbuilt transformers.

I’ve had a dig through the various Python APIs and I haven’t come across anything (doesn’t mean something doesn’t exist mind you!).

Each version of FME does come with the previous versions of the transformer, otherwise this would break the backwards compatibility of the software.