We recently (probably when we moved to the VPS?) broke the support for OPDS export. This has only ever been for French Wikisource, and it takes the contents of Catégorie:Bon pour export and builds an OPDS XML file. This file is used by readers in programs such as Calibre and FBReader, and by websites such as https://www.noslivres.net/, to syndicate Wikisource content. The URL of the ODPS file is https://wsexport.wmflabs.org/wikisource-fr-good.atom
It shouldn't be too hard to extend support to other Wikisources (they just need to designate a category to pull from).