It seems that there's no way to share a link to a folder such that an anonymous user with that link can then add files to the folder, is that right?
This seems like a pretty severe limitation. A "drop box" is a place for other people to drop files to you. For example, a drop box on macOS has, for many years, been a directory served up that has write-only privileges to unauthenticated users; they can't view the contents of the folder, nor delete any of the contents.
For my needs, it would be sufficient (although not perfect) for an anonymous user (actually some client API code I'm writing) to be able to add files to a given share link, without needing any other credentials (just the share link). It's okay if that link also provides view and delete capability for the folder, although not ideal.
Am I just missing something somewhere?