Plex quietly added the feature to its DVR service this weekend, which requires a Plex Pass subscription, in an update with several bug fixes. You'll have to manually enable the commercial-snipping in the DVR settings, but thereafter it will cut the ads out permanently, deleting them from the file itself. Better hope that it correctly flags material or the setting will cut out parts of your show. Also, be aware that "this process may take a long time and cause high CPU usage," according to the feature's description.