I remember someone asking about it maybe once or twice, but it is not a common thing being asked here. If that sounds easy, then provide a patch Or find/hire Java developer to add such thing. Clustering base has been added by JiveSoftware as a commercial plugin when they were still owning Openfire (open sourced since). A few devoted users have customized it to use free Hazelcast solution instead of commercial base. SSL is pretty much standard everywhere today and it has been added by JiveSoftware also. Of course, a few developers today are tinkering and updating it. But again, they are interested in that feature for their businesses/needs. If no developer is interested in email confirmation feature, it will never be implemented. No matter how easy or hard it is.
Now about the feature itself. You probably know that Openfire can't shoot emails just like that. Yes, there is Email settings and you can provide a SMTP server configuration there. So, you will need a SMTP server for that and one with good reputation (spf, etc.) so spam filters of your users won't filter its messages. Then the url link in such email. It should lead to some web service. In case of the forums this is rather simple. As web service is working directly with forums database, so it can update it when a user opens a link and it detects that. Web service can't have a direct access to Openfire's database. There is a Registration plugin for Openfire, which provides web registration form, which talks to Openfire via jsp pages (forms). I'm not a developer, so i'm not sure how it works exactly. There is also REST API plugin which can operate with Openfire via url commands. So combination of those two can be used for this probably. Though i don't like the idea of exposing Openfire's internal Jetty web service to the Internet (it is updated not so often and Admin Console itself has various security vulnerabilities).
Here, i have filed a ticket for this, but as i've said, with no interest from the developers (i mean not just ones working with Openfire, but globally) it won't get anywhere OF-1276