Found the below on UEFA website. It looks like they are emailing to confirm either way. Also says the ballot is being done per match, so those with tickets to Croatia v Scotland might still be in luck.
The results of the ballot will be communicated to every ticket buyer via email during May. In this email, ticket buyers will be informed whether their ticket(s) will remain valid or have been cancelled.
If your tickets have been cancelled following the reduction of the stadium capacity, you will receive a refund for the face value of your cancelled tickets.
Please bear in mind that, if you are successful in the ballot and your tickets are not cancelled, you will not be able to request a refund in case you no longer wish to attend the tournament.
IMPORTANT: Please note, that as UEFA will run the random ballot per match and per order, you might receive separate email notifications related to the result of the ballot for a specific match and these email notifications might be sent out at different moments.