google? signal desktop is not on android, is it? no, its on a pc, as could be a local llm. did i even say i did this before you wanted to assume that you are a better person because what your preferences should be universally obeyed?
Be angry then, IM communication is not a contract with terms unilaterly decided by one party. Or put a confidential signature with every message, see how well that works for you
there's a tool for extracting chat history from signal desktop, you could build a plaintext and attachment archive with that if it runs regularly on your pc and appends new chats from the last run.