Are decentralized stock exchanges legal?

The legal nature of decentralized finance is complex and sometimes vague. Laws guiding decentralized finance are those where related activities overlap with existing laws. In such cases, parties to the case refer to the law to seek recourse. Nevertheless, the jurisdiction of distributed nodes is impossible to establish, and neither can anyone be held for the decisions of a smart contract.

If you mean decentralized exchanges, then they are operating and do not have so much interference with the law if they ever do. They are businesses controlled by no single entity and can be registered as DAOs when the law captures the nature of what is going on around them. If you are concerned about the safety of these projects, then you need to do some background research on the founders, the project audit, and the objective of the project. If it is something you are interested in, you can join their Discord or Telegram community and ask further questions before investing.