The hastemplate field allows for arbitrary input since there are use cases for this feature like searching for pages that still contain a non-existing template after deleting it. When adding a non-existing template to the search field, there is no indication of whether it exists or not after the tag was created.
We want to visually convey that the user is adding a non-existing template to their search by marking the text in red like it happens for categories.
Screenshots of the upload wizard's behavior:
 There is a dropdown that suggests which categories exist, but you can also write your own category name that is not in the drop-down.
 The tags of categories without a file page have red font.
- We cannot use the category multiselect widget here, but we could look at it for inspiration.