It can happen that - for various reasons - some tag is deprecated, i.e., it is no longer supposed to be used. The reasons can be various. One possible reason might be that the area covered by the tag is no longer considered on-topic. Another possible reason is that the tag does not contribute to good tagging - maybe because it is ambiguous or very often misused.
As far as I can tell, when a situation like this occurred, the technical solution for this was usually that the information about the tag being deprecated was included in the tag-info.1
In my opinion, it might be reasonable also to prevent this tag from being added to new questions. A good way to achieve this is to blacklist the tag in question.
Blacklisting a tag does not mean all occurrences of the tag are removed.2 Blacklisting causes that the tag cannot be added to new questions and when question with the tag is edited, the edit cannot be saved with this tag still included. In such cases, a warning is displayed - either a generic message used for blacklisted tag, but it can also customized to include message specific for the given tag.
More details can be found on Meta Stack Exchange: What is a blacklisted tag?. As examples, I will also include posts from some discussion on other metas, which show a screenshot of the warning customized for a specific blacklisted tag - Mathematics Meta, Anime & Mange Meta and English Language & Usage Meta.
This means that by blacklisting a tag we can achieve that the tag is not added to new question. And by using tag-specific warning we can also increase the probability that a new user asking about something which is no longer on-topic is displayed this information when they try to add the corresponding tag.
- In future discussions about deprecating some tag, also the possibility of blacklisting the tag at the same time should be considered.
- The already existing deprecated tags - autograph-identification and fantasy - should be blacklisted.3
1I am currently aware of two tags on this site that are deprecated. One of them is autograph-identification (tag-excerpt/tag-info). The fate of this topic was discussed quite recently: Revisiting autograph identification questions (conclusion: off-topic). The other one is fantasy (tag-excerpt/tag-info) which was discussed here: Are questions about fantasy sports on topic?.
2In some cases, other additional steps might be reasonable. Often, blacklisted tags are also burninated. And in some situations it makes sense to mark clearly that the past questions are no longer considered on-topic by adding historical lock. But such steps should probably discussed on the case-by-case basis; I would like to concentrate only on discussion about blacklisting.
3Of course, if the consensus is that for each of these two tags, blacklisting should be discussed separately, I can make a separate post for each of them. I suppose in one case a suitable way might be posting a new answer to: What shall we do with questions tagged with the “fantasy” tag? For autograph-identification this would probably mean creation of a new topic.