SUMMARY Now that tags are more advertised in Dolphin (since 19.04.0), I thought that tag s could be better advertised in Folder View too. Specifically, I suggest a checkbox for "Tags" and a list of user-created tags to choose from, to be placed in "Configure Folder View" → "Address". It is already possible to show a tag by entering "tags:[name of tag]" in "Custom location", but a checkbox will make the feature available to more users.
This actually works already if you enter "tags:/<tag name>/" into the "Custom Location" text field or choose one of the tag items from the "Places Panel Item" combobox. But I agree that this could use a better, more discoverable UI.