In Editable Templates, instead of assigning components to the Parsys via Design Console, we use the policies to assign the components to layout-container. As you're using WKND Site, they've made the implementation via Editable Templates.
To make the component available in the side panel, your component should be part of a component group and the same component group should be allowed in the template policy. If it was working earlier and now you do not see the component in side panel, that means the template policy is was not part of the code base and it was created directly on AEM instance. Post the deployment, the template policy got overwritten and which is why you do not see the option.
To re-enable it you need to go the template editor, and allow the component group in the template policy.
Now for Design mode, it is only available for static templates. Templates that are created with editable templates should be edited using the template editor which will be using the template and component policy.