If I am using, say, Signal, I do not want it to be possible to send me an unencrypted message at all. That philosophy of security sounds likely to run afoul of this legislation.
Signal can give you the option to disable that. They just cannot force that decision on users.
Security needs to be a default though. It will be a serious problem if this law ends up being interpreted as requiring encrypted messaging systems to accept insecure messages by default.