https://www.irs.gov/e-file-providers/free-file-fillable-form...
https://www.irs.gov/e-file-providers/free-file-fillable-form...
This is not a verbatim quote, but solar to what you have to do now to calculate health care insurance subsidies.
As a bonus, the spreadsheets are generally reusable, after checking to make sure there are no changes.
I failed to copy my self-employment tax back to my 1040 for 2020, despite having calculated it correctly. No warnings, the form filing went smoothly, and 4 months later I got a notice from the IRS.
It's insanity.
My suspicion is that it is deliberately deprecated due to pressure from for-profit tax prep companies.
PDFs aren't exactly used in a standardized way, so trying to make this work for even a fraction of the numerous tax form issuers requires a lot of work, and a lot of ongoing maintenance. You'll need to prioritize the actual tax forms, and annual updates to those first.
Hypothetically, even if an API existed, building the integration in your own app also costs FTE hours that are likely drawn away to higher priority tasks.