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/

Update to the Syndication Links plugin for WordPress for Custom Endpoints (BoffoSocko)

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