Have you been using Gutenberg, only to notice that your custom taxonomies have disappeared? It's true, they have. Sort of.
Don't worry. Your custom taxonomies are still there. Gutenberg just needs to be told about them. Gutenberg pulls custom taxonomies using the REST API. Never used the REST API? Neither have I (yet). But enabling it for your taxonomies is actually really easy.
Go to your register_taxonomy call and add the following to the declaration:
'show_in_rest' => true,
That's it. Seriously. Do that for each of your custom taxonomies and you're good to go.