The ElsterFormular software doesn't know about the 10 year rule.
Good news, I have some new information:
There is a last step the Finanzamt performs in the case of international cases. In the last step, they go back to the German income tax code (EStG), and if an income that would fall under Progressionsvorbehalt because of a double taxation agreement would not cause tax if it was German income, they don't apply the Progressionsvorbehalt.
So you're in the clear, the profit from the sale does not cause any tax, not even through Progressionsvorbehalt, since the flat was bought more than 10 years before the sale.
So it's your choice:
don't fill in Anlage SO at all, or
fill it in, but also fill in on the Mantelbogen ESt1A in line 98 that you have additional details to tell them, i.e. that you will send them by snail mail the copies of the proof that more than 10 years gave elapsed. Marking line 98 will mean that your tax return will be processed by an actual person who will "know" about the 10-year-rule and not automatically by a computer, which may not have been programmed with this rule.
When the Bescheid comes, check that this profit didn't raise your tax!