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.

Just like me!  I really should learn how to put my right foot down for when that’s a better idea (dog on left, say) but I just can’t!

Right-handed, left-foot down. I think that's because I mount/dismount on the left side of the bike as I tend to lean it up against something on my right. Since I ride on the right-side of the road here in the US, objects to lean against are normally on my right.

Left handed, left foot down. I don't think there's a correlation for me, it's more practical than that. I cycle in the UK so left foot down lets me rest on the kerb in a more comfortable, upright position. And if I'm on the offside of the carriageway, waiting to turn right, it leans me away from oncoming traffic rather than towards it.

Left-handed for fine motor skills such as writing, eating, toothbrushing. Right-handed for gross motor skills such as batting or throwing a ball or fisticuffs. Left-legged because that’s how I mount and dismount, as well as using my right leg for the power stroke. 

If I remember back through the mists of time, we were taught to mount from the left. Maybe because of the chain and derailleur being on the right side, but I also think it may go further back to when bikes were new, and continuing the rule about mounting your steed from the left side.