The Jekyll Core team is considering an idea to drop support for mixed-case URLs. We have not finalized it yet, though.
As things stand now, Documents in a Collection generates URLs with the categories and title lowercased but standalone Pages generate mixcased URLs. In other words,
./Movies/_posts/2019-05-16-EndGame.md will generate
./Movies/Avengers.md will generate
The Core Team proposed converting all URLs to lowercase to remove the above discrepancy and because not all platforms handle mixcased paths / URLs in the same fashion.
However, before we finalize this, we’d like to get your feedback on this move especially since existing mixcased URLs will error 404 with this.