iOS 13 (and web interface): Cannot share an iCloud folder
iCloud folder sharing was included in some of the early iOS 13 beta releases but Apple removed that feature in Sept 2019.

As of now, iOS 13.1.3 is publicly available stable version and Apple has not added iCloud folder sharing back.

Some sites, like this 9to5Mac site as well as this Apple macOS Catalina features page are reporting that it should be available again in the "Spring of 2020".