Hi guys,
In older versions (e.g 2.x.x) you could add sub-menus to a dropdown, but in version 3.x.x this feature is missing.

This feature was purposefully removed by @mdo as he feels that submenus aren't great UX, particularly on mobile.
What's the alternative?
I believe there wasn't any one-size-fits-all suggestion beyond reworking your information architecture to not require nested menus.
You can also ask him yourself via Slack: https://github.com/twbs/bootstrap/blob/master/README.md#community
The only issue with multi-level menus I see when browsing the other issues is that hover is not possible on mobile. IMO this can be simply resolved by also expanding on click, not only hover. Best of both worlds IMO. Single-level menus are IMO crap for navigation. I hate when i have to click multiple times to reach some part of the web site.
I'm attempting to use a sub menu in a Dashboard Application and at the moment bootstrap seems to be the best solution, but it's resulted in fair bit of hacky code seeing as the sub nav isn't there anymore. I'd like to see it reintroduced at some stage if it's still up for discussion. It definitely has it's place in applications, if not in normal websites.
Most helpful comment
The only issue with multi-level menus I see when browsing the other issues is that hover is not possible on mobile. IMO this can be simply resolved by also expanding on click, not only hover. Best of both worlds IMO. Single-level menus are IMO crap for navigation. I hate when i have to click multiple times to reach some part of the web site.