If you are legally married, you can file as "married filing separately" any time you want, you don't need any proof.
If you want to file as "head of household" you must have lived in a separate home from your spouse every day from July 1, 2019 to 12/31/2019 (to file as HOH on your 2019 tax return). You don't need any proof of a marital separation, what counts is where you and your spouse physically lived and spent the night. If you spend the night together in the same home any time after July 1, you can't file HOH even if you are legally separated.
If you want to file as "single", you must be unmarried according to state law. There are a very few states and very limited circumstances when you can be legally separated and file as single. This mainly applies to older laws and older times (say, the 1930s) when it was almost impossible to get a divorce and married couples might separate but remain undivorced for years or decades. This kind of legal separation is still on the books in New York state, but it requires court approval and costs as much as a real divorce so most people don't do it. A voluntary separation, even if approved and supervised by the court, is not enough to be considered legally unmarried. Other state laws are similar. I have never heard of anyone in the 1990s or 2000s who is still legally married (not divorced) but was allowed to file as single under a separation. But you would need to check with an attorney familiar with the laws of your state. Even then, you don't send proof to the IRS unless you are audited.
If you want Turbotax to give you a letter proving you filed a tax return as "married filing separately", just give them a copy of your tax return or your tax transcript from the IRS. Turbotax does not issue such letters.
