Can I adjust past tax returns to include cash charitable donations made and not taken- since this year they are no longer allowed?

You will have to amend your past year tax returns to claim the charitable deduction.  You have three years from the date you filed your return or two years after you paid the tax due (whichever is later) to file an amendment.  See How do I amend my federal tax return for a prior year?

There are no recent rule changes on substantiating donations. 

You may not deduct any cash donation of any size, unless you have a written receipt. This rule has been in place since at least 2018, unless it was changed in 2015.  Any prior year contribution that you have a receipt for was already deductible in that prior year, and any prior year donation that you don’t have a receipt for is not deductible on an amended return now anymore than it was originally. 

Non-cash items dropped off at an unattended dropbox must be substantiated by certain written records that you must keep.  If you have those records from prior donations, they would’ve been deductible in the prior year. If you don’t have those records, they aren’t deductible now, even if you want to amend your return.

in short, any donations that you don’t have proper records for have not been deductible since at least 2018. There has been no sudden change. Any donations you made in the last three years would have been deductible at the time if you had the records, and are not deductible now, or then, if you don’t have records.