There are a lot of ingredients that are hard to find at the supermarket and don't have tags. Should these kinds of rare ingredients be added to the tags list?
- Caciocavallo Podolico Italian cheese
- Beluga caviar
Durian seems like a reasonable tag, mainly because it's not actually a rare ingredient. (Sure, it's hard to find in some parts of the world, but it's a widespread thing that plenty of people will have had.)
Beluga caviar and Caciocavallo Podolico Italian cheese both look way too specific. To be useful, tags have to be things that'll get applied to multiple questions. (In fact, the site even automatically deletes tags that don't get used enough.) caviar is a good tag. So is cheese - and some larger subcategories of cheese are good too. But tags for specific varieties don't really add anything.
Tags serve a fairly specific organizational purpose: they're for helping people search for questions, whether it's to find related things to read, or to find questions they might want to answer. (They also sometimes help disambiguate questions, e.g. using indian-cuisine to specify that the question is talking about Indian curry as opposed to Thai curry.)
If a tag is too specific, neither of those is true: there are no other questions to search for, and there's no need to disambiguate since the question will already have done that.
Finally, note that the best way to add tags is via edits. If you don't have the privileges to edit (or suggest an edit) to add a tag, and you really think it's a good idea, you can leave a comment on the question. If someone who has the necessary privileges agrees, they can do it for you.
If on the other hand you're asking about adding a general tag for rare ingredients, that'd be an example of what we call a meta tag, so it's not the kind of thing we'd like to add.
Remember the organizational purposes of tags mentioned above: searching and disambiguation. Neither of those things really seems to be true about rare ingredients. Instead, it's sort of a more broad categorization that doesn't really help people search - and those are the meta tags which we avoid using as tags.
An additional strike against this tag is that it'd tend to be very inconsistently used. It's not the kind of tag people would think to add to their questions. It's also not clear when you should use it - you have to figure out how rare something is, which is extremely variable depending on you live. Most things that are rare where you live are probably common somewhere.