Apple has an open source project that has the same goals (documenting password rule quirks), but your idea to crowd-source the collection of rules is interesting.
I will list below several other Feature Requests related to the ultimate goal of being able to generate random passwords that are compliant with different websites’ idiosyncratic password policies. Some of these should probably be merged, but your suggestion may be sufficiently different that it should stand as its own request. I’ll let you and/or the mods decide about that. Here are the related topics that I could find: