I think it would really help future users/maintainers if you explain the rationale. Basically, you want `matchNullFetch = true` if you are using an implementation of Union that preserves order. If Union does not preserve order, the limit is doing no useful work. If the Union does preserves order (and merges on the sort key), you are still doing useful work if fetch is null, because at least you have done the sort.
I can see the concept of "sort-preserving union" being useful in some other rules, so I don't want it buried in an obscure flag.