“We’re aware of reports that access to Signal has been blocked in some countries,” Signal says. If you are affected by the blocks, the company recommends turning on its censorship circumvention feature. (NetBlocks reports that this feature lets Signal “remain usable” in Russia.)
You can just as easily identify servers of a decentralized platform and block them. The disadvantage of a central service would come into play if say the US were to intervene, though Signal has already said they would move abroad if that was the case. For network level blockage it makes no difference if the service is central or not