Tags functionally behave as categories, but with a different format than categories. In that sense, it would be natural and useful to set tags as accessible only to certain groups, and in the same line, benefiting from the fact that tags can be set as accessible only from certain categories (essentially as sub-categories), it would be useful to link chat channels to said tags (automatically adding to said channel only the group of people that haves access to said tag). This will allow to easily assign people to more specific social entourages within a large forum, while keeping the forum format as the macro cohesive factor. I think this will allow to profit more from the order that the chat format can bring to ephemeral communication while keeping balance with the forum format.
You can do that with tag groups I do believe.
Pretty sure this is also possible with tag groups. (If not tags themselves outright)
The first thing you mention is possible only for groups of tags, not specific tags. For the second one I meant to say that since it’s already in place, the last remaining and natural thing to do is to be able to link a chat channel to a specific tag.
It could be done with groups that consist of one tag, but I see your point here.
Could you explain your use case here? I can’t see a reason why you’d link a tag to a chat channel.
Actually not at all. Yes, tags are kind of another semantic way to organize content, but tags are fundamentally different thing than categories.
That thinking error are raised from very common mistake when admins have used categories when they should use tags.
Tags are connecting same type of content horizontally, and then an user doesn’t see content from limited categories. Problem solved 
I’m sorry for the delay in my answer. My use-case is the same as the one that prompted chat channels to be associated to categories. The reason I consider the same should apply to tags is because I consider tags a special type of category. A “meta-category” perhaps, for I think tags are more flexible and are already capable of category-like functionality.
I’m sorry for the delay in my answer. I understand what you mean because I treated tags the same way at first, but soon realized that in practice tags are not different in nature to categories. They can do things categories can’t, but the opposite is not true as you suggest: tags can do what categories do as well, and it’s so natural that it happens all the time. It’s so common that Discourse had to implement functionalities to deal with that, resulting in “category specific tags”, and “tag groups accessible to certain groups”. By combining both you have just recreated sub-categories. Just because tags can show information from different categories in a disordered way doesn’t mean they are “avoiding limits”. A more ordered way to show the same information would be to group the posts from a certain tag by the category they come from and lo and behold you have “rows with featured topics” just like if you where using sub-categories. That’s actually a nice example because it shows that by vindicating tags as a type of category you actually have more functionality to display information than we have today by strictly differentiating tags from categories.