Nice example how custom POSSE providers of the Syndication Links plugin are supposed to work: https://boffosocko.com/2019/02/16/update-to-the-syndication-links-plugin-for-wordpress-for-custom-endpoints/
Custom POSSE Providers example for Syndication Links plugin
David Shanske has recently updated the Syndication Links plugin for WordPress that now allows users to add custom syndication endpoints to their websites so they can actually syndicate their content