al-Qaeda reportedly favoured communicating in the clear with obfuscated messages, attempting to make their communications become a needle in a haystack of mundaneness that no one would raise an eyebrow to[1].
In World War 2, the United States hired native American speakers to communicate in the clear, knowing that the Nazis would have no comprehension of the languages spoken[2]. Use of obscure languages, local dialects and local euphemisms has been widely used by drug cartels[3], parties to violent conflicts, etc.
Criminals have been known to join and communicate via obscure online multiplayer games under the assumption that no one has gone to the effort of reverse engineering a proprietary game protocol and the developer hasn't gone to the effort of implementing recording of or monitoring of in-game activities that could be used for communication, including painting words on a surface in-game, naming objects in-game, etc. Online multiplayer games also provide a plausible reason (cover story) for two people to communicate with each other and yet claim they don't know each other in real life--a feature most encrypted messaging protocols can't guarantee.
Encryption backdoors are unlikely to be a concern to adversaries we should be most worried about. History shows they have tended to assume that backdoors exist and that use of encryption tends to draw unwanted attention.
[1] https://en.wikipedia.org/wiki/Obfuscation#Secure_communicati...
[2] https://en.wikipedia.org/wiki/Code_talker
[3] https://publicintelligence.net/dea-drug-slang-code-words-201...