Even if a plugin would have mono output, its output would usually end up in a stereo audio file. And even if you'd release strictly mono audio, your audience will probably listen to it over a pair of speakers anyway. So what would be the point of a mono-output plugin, unless it's specifically used for processing mono input?
Btw, monophonic has nothing to do with mono vs stereo.
Edit: I'm interpreting "mono" as "single channel" here. Not as "identical signals over both the left and right channels".