My guess would be that because this is "guidance" rather than a law or "bill" then this is not ex post facto application. In other words, the law is and aways

My guess would be that because this is "guidance" rather than a law or "bill" then this is not ex post facto application. In other words, the law is and aways has been that one should pay the appropriate taxes. OTOH, guidance informs the IRS how to interpret market activity in the past and classify it as legal or illegal.