I haven't said that it is restricted to XP SP2 (for a start I'm on SP3), just that things have changed in the latest release of avast from 2015 R4 on how the https scanning is achieved in XP and this still needs more work.
Avast hasn't changed the system requirements for some time, certainly not by any great margin. As for SP2 whilst it is still supported, there are going to be areas that aren't supported in the that OS because it isn't possible and the new NG (virtualisation function) is just one thing of the top of my head that isn't available in XP of any SP variety.