I’m creating a custom field using the Post Object field type and am filtering by post type. But I wondered whether it would be possible to filter by page template instead of post type?
Thanks
No, this is not something that ACF will do. There is nothing that will let you add new filter types to a post object fields. You would need to build your own custom field type based on the post object field type. https://www.advancedcustomfields.com/resources/creating-a-new-field-type/