When will TT treat 1040-line 10(donation allowed by CARES Act) correctly for filers who take standard deduction? TT allows input, but does not deduct 10c from AGI.

No one in this users forum knows for sure, neither do most/any of the TTX reps that might poke in here occasionally.

I suspect it would be implemented in the next 3-4 weeks.  Even so, the IRS won't start accepting tax returns until ~3rd week in January, so there is no particular rush. 

 ( That is , unless the current president actually signs the budget bill Congress sent to him last week...if he does sign it, then the IRS start date may be delayed even further....but that's just a guess )

TurboTax does deduct line 10c from AGI now. Are you looking at the actual Form 1040? Does the contribution amount appear on line 10b?

If you entered the contribution under Charitable Donations in the Deductions & Credits section, TurboTax doesn't make the entry on Form 1040 until you finish that section. After you click "Done with Deductions," a few screens later it tells you that you are getting the standard deduction. When you click Continue on that screen, the next screen tells you that it has added the $300 contribution to your tax return. Only at that point is it deducted from your AGI.

Most of the CARES act changes have not been added to TurboTax yet because the IRS is still working on the form changes and instructions for 2020.   Many of the new forms are still in the draft state.