Is there a reason for this tag to be missing?
Am I wrong to assume that connection strings are on-topic here?
No tag is ever considered "missing" by a fiat decision (unless it's a synonym in which case it still "shows up").
The reason tags don't exist is because nobody is using that tag. The same goes for tags like
ravendb (a NoSQL DB engine with ACID support) that would be very on topic here but which nobody has yet asked about.
Stack Exchange is a community driven site, and we let the community help us guide things. One of those things is the creation of new tags, just like others are up and down voting, the ability for anyone to edit, and the ability to close questions which just don't fit our particular site.
In this case, you need 300 reputation to create tags (see: https://dba.stackexchange.com/privileges/create-tags) but once a tag has been created/added you can use it on your own questions. The reason for the limitation is just to keep people from creating junk tags which aren't related to the site.
Ask great objective questions and add the tag. And when you don't have enough rep to add the tag, you can ask someone in chat to add it or you can flag the question for moderator attention and tell us that you need to add that tag, and we'll add it for you.