If the plugins are missing from Lightroom, first verify to see if they are installed and enabled in Lightroom.
In Lightroom goto the File Menu>Plugins Manager
Do you see ON1 PHOTO RAW 2019 listed and is it enabled? If it is listed but disabled, just click the enable button.
If it is not listed, click the Add button here and navigate to your Applications folder> ON1 PHOTO RAW 2019>Plug-Ins> ON1 PHOTO RAW: Select the ON1 PHOTO RAW 2019.lrplugin file and click the Add Plugin button. Then click Done.
You should then see the plugins listed under the File>Plugin Extras menu.
Path: Applications Folder>ON1 PHOTO RAW 2019>Plug-Ins>ON1 PHOTO RAW: ON1 PHOTO RAW 2019.lrplugin