There's a small market for it. I think in the vast majority of the situations, it would be better for an aspiring lefty to simply play righty.
If you have any expectation to play orchestral work, don't even think about it. (see exception below)
AFAIK, all of the lefty uprights I have seen offered were entry-level instruments. Keep in mind that an entry level upright is not exactly inexpensive. Makes a bit more sense for someone who wants to play lefty bass guitar, as lefty instruments are available at whatever price you want. Playing lefty bass guitar does still limit your options, but not as significantly.
I would be concerned with the ability to progress and move on to a fine quality upright. A serious player is likely to become dissatisfied with an entry level upright very quickly. When the time comes, IMHO it's usually best to play a bunch of basses to find the characteristics and personality that fits you. If you need a lefty upright, chances are you will need to commission a new build. Not sure how receptive a master builder will be.
I concede that some people could be better off with a lefty. However, IMHO those who go on to be accomplished musicians may be put at a serious disadvantage from starting lefty. No one knows the future.
Exception: Niagra Symphony [Filip Stasiak (lefty upright]
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