Rate-limiting is a real problem for it, and one reason it might not be feasible as a product as some APIs still rate-limit based on IP, so for those there would have to be some randomisation of IPs used.

I'm hesitant to share it as I know it's quite buggy and a bit too simple at the moment. I've not tested it with enough variety of APIs and use cases. Plus the whole UI needs deleting and starting all over again, but built properly! With all those caveats, it's https://apiarchive.com/