@Pascal

You could find it on the field website_meta_keywords of the model ir.ui.view (table ir_ui_view) specifically for the page you use the promote form, for example if you do it on the homepage this website_meta_keywords values saved are on the ir.ui.view record for the Homepage view. You could also issue this SQL query who will discover those records for you:

select * from ir_ui_view where website_meta_keywords is not null;