All yeasts?? I don't think that such a level can be stated. However, its been apparent that many lager yeasts are known to perform better in lower calcium water. That's probably due to the liklihood that that yeast variant has acclimated to the low calcium water that was used for its historic beer production.
There may be some ale yeast variants that would also prefer lower calcium content. For example, there are some yeasts that are known for flocculating too early. Since increased calcium content is directly correlated to how well yeast tends to flocculate, it is feasible that those 'early flocculators' should be used in worts produced with low calcium water.
There is no minimum calcium level in water needed for yeast metabolism since the malt provides all the calcium that the yeast need for their metabolism. But there are other reasons to have calcium in your brewing liquor: enhanced enzyme activity, oxalate precipitation, pH reduction. But probably the most recognizable reason is for the delivery of chloride and sulfate for beer perception.
I suggest that any brewer that has problems with premature yeast floccing, look at the calcium level they have in their brewing liquor and try reducing it, if possible.