The words ProConnect were not always this online tax product you are using. ProConnect Tax Online was Intuit Tax Online. ProConnect was more like office management. I think you found a reference to this:

https://webcache.googleusercontent.com/search?q=cache:S4uuuwDJ3-MJ:https://proconnect.intuit.com/sup...

It used IRS Application-to-application. I think that's from 2018? 2016? I've never supported a client hub or document management. I support and code interfaces for Quickbooks and Lacerte, mostly, and computers in general for my clients. Someone might remember using that tool. I think it was part of document management, not tax prep.

Lacerte has an SDK to send it field data for forms. Nothing here reads an IRS transcript into a tax product form, as far as I know. That's not formatted. You can get brokerage data that way, though. .txf and .xml can be used.