People within the occult movement tend to be rather pessimistic with regards to the relevancy and integrity of Satanism in the 21st century. Well, the question isn’t exactly as simple as whether or not it is relevant. We need to first take into account what Satanism is.
I consider the only valid definition of Satanism to be whatever works for the particular Satan, or the adversarial person. In other words, if you take on the Devil’s name and play the Devil’s game, Satanism is whatever it is to you. It’s not up to me to determine your life for you. If this is what we understand as Satanism, then yes, it is and always will be relevant. The only problems would have to arise from the particular person and their personal shortcomings.
There’s, of course, another way to look at Satanism, a way I violently disagree with, and that’s to look at a book, a guru or what have you and just calling that Satanism. If that’s how you look and it then yes, sooner or later it’s going to be outdated and irrelevant. That’s also the biggest problem with mainstream religion: dogma. Dogma will always be outdated and struck aside at some point.
That said, I do consider Satanism very relevant, I think it always will be. What you choose to call it is a separate issue entirely: what matters is the essence. It’s the Satanic spirit of progress, strife and antinomianism that matters.