I'm unconvinced we need an iframe tag, because iframes are not an easily defined WordPress development issue.
When iframe questions do come up, they're usually in the context of something else that we have a tag for, such as visual-editor or tinymce in the case of this question, menus in the case of this one, and widgets in the case of this one (using the three examples cjbj provided).
There is likely to be quite a bit of disambiguation between the various uses of this tag - eg. 'my iframe tag got filtered' is a completely different category of iframe question than 'I want to show a portion of my WordPress menu in an iframe'.
Because of this disambiguation:
- it's likely that iframe questions will require expertise in completely different parts of WordPress;
- and, iframes are unlikely to be the core issue being asked about - the core issues in the examples above are filtering of disallowed tags, and displaying WordPress content outside of the WordPress environment.
I think because of this a tag for iframes wouldn't serve an easily definable purpose, and could possibly end up as a candidate for removal in the future.