medrxiv was very useful for keeping the various COVID-19 related preprints from completely swamping biorxiv, especially once biorxiv started aggressively rejecting them.

Sadly I couldn't find a public API for chemrxiv, but would be happy to be proven wrong!

Here it is:

https://chemrxiv.org/engage/chemrxiv/public-api/documentatio...

Thanks!

There is also engrXiv, which has an OAI endpoint. https://engrxiv.org/oai?verb=ListRecords&metadataPrefix=oai_...

Amazing!