twistd itself was converted over -
https://twistedmatrix.com/trac/ticket/8235 - but of course that's mostly from the consumer side rather than emitting logs. It shouldn't be too complex, honestly; just get rid of all manual string formatting, and convert any %()s format strings to {}. The testing support is the same as for the old logging system (add a global observer, remove it in an addCleanup, assert about the things it caught) because it's still just key-value pairs, they're just better-defined now.