It's a few years old, but you can read this topic to get a sense of the status at that time:

https://proconnect.intuit.com/community/proseries-tax-discussions/discussion/txf-tax-exchange-format...

Most everything, even the IRS, has moved to XML, of course, if there is much precision with complexity. Otherwise, csv is often an interim stage.

I'm in the Intuit Developer community and I've never seen anything other than the Lacerte interface for the tax prep products, the Trial Balance import for ProSeries, and the Smart Vault/DMS. There even is an SDK subforum here for the Lacerte forum, but it doesn't have much traffic.