SFTPGo
Fully featured and highly configurable SFTP server with optional HTTP
Virtual folders are supported: a virtual folder can use any of the supported storage backends. So you can have, for example, an S3 user that exposes a GCS bucket (or part of it) on a specified path and an encrypted local filesystem on another one. Virtual folders can be private or shared among multiple users, for shared virtual folders you can define different quota limits for each user.