Tandems are sometimes set up with the timing chain on the left side.  I can’t give you percentages, but we have 3 tandems, and 1 is “sinister”.  

Hi Graham.
Yes, there were. And they still are, though thankfully (because such things should be suppressed in a well-ordered world) not in large numbers.
Track teams use them for reasons they can justify to themselves.

See https://www.cyclingweekly.com/news/japan-unveils-new-olympic-track-bike-with-left-sided-drivetrain

My situation is similar. I always put my left foot down and mount from the left side, but I am right handed. The cartilage in my left knee is long gone. I put my left foot down and leave my right pedal up. That way I can use my right leg for that first power stroke.

The last time I mounted my bike from the right side, I was in grade school. 

I am also finding as I pass through my late 60s that I occasionally clip my rack or saddle bag when getting on and off the bike, as well as my top tube when getting off my Tour Easy recumbent. Flexibility ain't my middle name.