/usr/include/ags/plugin/ is in libags-audio-dev 1.4.24-1ubuntu2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/ags/plugin/ags_base_plugin.h
/usr/include/ags/plugin/ags_dssi_manager.h
/usr/include/ags/plugin/ags_dssi_plugin.h
/usr/include/ags/plugin/ags_ladspa_conversion.h
/usr/include/ags/plugin/ags_ladspa_manager.h
/usr/include/ags/plugin/ags_ladspa_plugin.h
/usr/include/ags/plugin/ags_lv2_conversion.h
/usr/include/ags/plugin/ags_lv2_event_manager.h
/usr/include/ags/plugin/ags_lv2_log_manager.h
/usr/include/ags/plugin/ags_lv2_manager.h
/usr/include/ags/plugin/ags_lv2_option_manager.h
/usr/include/ags/plugin/ags_lv2_plugin.h
/usr/include/ags/plugin/ags_lv2_preset.h
/usr/include/ags/plugin/ags_lv2_uri_map_manager.h
/usr/include/ags/plugin/ags_lv2_urid_manager.h
/usr/include/ags/plugin/ags_lv2_worker.h
/usr/include/ags/plugin/ags_lv2_worker_manager.h
/usr/include/ags/plugin/ags_lv2ui_manager.h
/usr/include/ags/plugin/ags_lv2ui_plugin.h
/usr/include/ags/plugin/ags_plugin_factory.h
/usr/include/ags/plugin/ags_plugin_stock.h