I think Open Source works well when the underlying work is fun and the software is somewhat timeless to minimize ongoing workload. Tax software by its very nature is none of those things.
Honestly I think OSS is great but this is more fitting for the IRS to do itself rather than hoping that a bunch of lawyers/tax professionals/financial programmers suddenly decide they want to spend their evenings writing the most boring software imaginable.
There is no way I am going to get a bunch of accountants and tax lawyers to write a ton of code each year for free, but ask them to build a plain English questionnaire for a form as easily as you would create a survey in SurveyMonkey? Maybe.
for an actual e-file it's a lot more complex, is called MEF https://www.irs.gov/e-file-providers/modernized-e-file-progr...