^^^ Ok, but what is to prevent financial institutions (like Fidelity or Vanguard) from rolling out software that CAN figure out the pre-tax portions, and therefore allow you to convert next year? Or, am I missing something?
By law (somewhere in the IRS code, which is Title 26, United States Code), Roth conversions from traditional IRAs which have after-tax contributions have to be made on a pro rata basis. So if 10% of your traditional IRA is after-tax, and you convert $100, then $90 of it is pre-tax and $10 of it is after-tax. You, me, the IRS, Fidelity, Vanguard and everyone else has to follow the pro rata rule (or break the law). Specifically this means you can't just tell Fidelity or Vanguard to only convert the $10 of after-tax money and leave the $90 in the traditional IRA. This is true even if you have all of your pre-tax money in one traditional IRA and all of your after-tax money in another traditional IRA.
If this law passes, then that $10 of after-tax money cannot be converted to a Roth. You, me, the IRS, Fidelity, Vanguard, and everyone else has to follow the "after-tax money cannot be converted to a Roth" rule (or break the law).
It's not a matter of the accounting. Vanguard and Fidelity can definitely write software to keep track of the pre-tax portions. But I seriously doubt Vanguard or Fidelity would break the law with you, me, or anyone else (again, assuming this passes, and assuming we're talking specifically about 2022 Roth conversions where someone has after-tax contributions in their traditional IRA).