Actually it’s not true for iMessage. Messages switches to SMS/MMS if the phone doesn’t support iMessage or has it disabled, but if it’s an iMessage it’s an iMessage—encryption and all—and you can see in advance if that is the case.
That said I don’t know what Google’s messenger actually does when it drops the encryption on RCS messages. Is there a visual distinction between encrypted RCS and unencrypted RCS on the sender end if it is just dropping the encryption on the receiving end as a fallback?
If you actually care about e2ee you'd use Signal.