You need Industrial Revolution to play Socialism.
Say I'm eligible for that, and I duly become Socialist.
Thereafter, I lose some levels (due to Civil War perhaps), and my level marker goes below the blue marker for Industrial Revolution.
Am I still socialist ?
Is the ruling the same for artefacts, eg Satellites needs Radio ?
There is no question that, as soon as you adopt a religion or government, that you do keep it regardless of what progress level you go to. I asked Harry about obtaining a religion or goverment an Age earlier by virtue of the "Exotic Artifact" event. His answer was that, once any nation has that religion or govt, anyone could adopt it, regardless of Age. I know that this digresses a little from the original question, but I feel that it is a fuller explanation of the details.
However, my feeling on blue artifacts is as follows:
Case 1) You actually are the owner of the green portion of the blue artefact upon the empire in question: you still get the benefit of the artifact.
Case 2) You only had the capabilities of the Blue artifact by virtue of being at the Progress level that is beyond the marker on the Progress Track: you lose the benefit of the said artifact if your empire is reduced in Progress Level to one that is below the position of the marker on the Track.