Fwiw I was shocked when I saw the t/f T/F kluge when I went to work on jsonlogging. That's the kind of dead-end short-sighted hack that just lays traps and barriers for future hackers to have to clean up before they can do the work they want to do.
Please just put a "format" field (or "channel" field -- the logging daemon doesn't really care what format) with a list of defined formats that can easily be extended in the future. If you want to steal the high bit for "is last" and only allow 128 values instead of 256 so be it.
