I would love to be able to understand the best practice and guide my UX decisions based on crowdsourced user analysis.
You could create it :)
Are you talking about something like this? https://www.goodui.org/ if not... lets make it.
You can't learn best practices from others' AB tests results. Anyone with MVT/AB experience will tell you that you shouldn't copy other tests because your context is different, unique – what works for them don't have to work for you (and vice versa).
My recommendation: do tests on your own, learn from results and get to know your users.
I agree, every product is unique (kind of) and have its very own context. But some UX part or UI pieces can be similar based on the same targeted users, same usability dynamic etc. I believe we can understand and create global patterns and learn from the others (otherwise, every websites would using its own paradigms).
I would love to try a platform where I could search for "Log in icon", "sign up form" etc, and read quick study cases with a bit of context in order to guide my design decisions (and spare some time).
Here you go guys: https://vwo.com/resources/
I cannot tell you enough how many tests I have run with this little gem.
https://whichtestwon.com/ is the closest but its so ancient and they only let you see 1 a day unless you pay. :(