No legislation has been passed and nothing has been signed by the President to change the tax code.

When and if this legislation actually changes the tax code the IRS will write the rules and the procedures and post those on the IRS website.

Correct.  At this point, it is just under discussion.

I'm pretty sure this will not be retroactive. It applies to UI payments to you in 2021. UI will be extended to September.

The change applies to tax year 2020, not tax year 2021.

Once the President signs the law, the IRS must develop procedures to implement it. Then TT has to program, test and get the IRS to approve its programming.  All that may take time.

The details of how this will work have not been sorted out yet. Here is a blog from TurboTax that will be updated with the latest news.