create a tag when a new tag is present in the submitted question.
delete a tag automatically when no questions are assigned such a tag.
current: tags will not be deleted by itself.
In my perspective, tags are useful when we want to filter questions. Also getting a list of tags helps prevent generating similar duplicate tags when creating a new question.
However, deletion of a tag is not a daily issue. There might be sometimes you want to delete a tag, but most of the time you only add new tags, and it is extremely easy to do so when creating questions. If the tags does not get eliminated when all questions it assigns to have been deleted, it will end up with a long list of unused tags, which are useless filters (filter nothing), also it disrupts the person who wants to create a new tag.
In my perspective, tags are useful when we want to filter questions. Also getting a list of tags helps prevent generating similar duplicate tags when creating a new question. However, deletion of a tag is not a daily issue. There might be sometimes you want to delete a tag, but most of the time you only add new tags, and it is extremely easy to do so when creating questions. If the tags does not get eliminated when all questions it assigns to have been deleted, it will end up with a long list of unused tags, which are useless filters (filter nothing), also it disrupts the person who wants to create a new tag.