That depends entirely on the need. I would bet that any sort of decentralized chat system communicating to nonstandard servers would be closely scrutinized.
For one-to-one communication, ideally I'd set up either some sort of special code with the receiving end and just use http. If more information relaying is needed, a one-time pad would be good. I'd try to keep the messages short in case there's a hole in the system somewhere. Again, depending on your needs, relying on one protocol like matrix or pgp could be risky. Good OPSEC can make up for a leaky security system.
For one-to-many communication, proxies and device disassociation are priority above all else. You can assume interception of those messages generally.
For one-to-one communication, ideally I'd set up either some sort of special code with the receiving end and just use http. If more information relaying is needed, a one-time pad would be good. I'd try to keep the messages short in case there's a hole in the system somewhere. Again, depending on your needs, relying on one protocol like matrix or pgp could be risky. Good OPSEC can make up for a leaky security system.
For one-to-many communication, proxies and device disassociation are priority above all else. You can assume interception of those messages generally.