2016 ACA Penalty refund (?) will expire April 15, 2020. You need to file a protective claim before April 15, 2020 to keep the return from 2016 open.  2017, 2018--maybe they will have determined if it was unconstitutional and you have time to amend.  I would wait a while to file the protective claim from 2016 in case it is resolved, but the supreme court doesn't want to rush on the decision.