I think you have to be careful with what you mean by “supported.”
The supported versions of Asterisk are 11 and 13, with 11 going into security fix only mode pretty soon.
By supported, we mean that if you have a bug in 11 or 13, the developer community will triage and attempt to provide a fix. As this is an open source project, there’s no guarantee of a fix being provided by anyone in the developer community. Of course, because it is an open source project, you can always fix it yourself and contribute the fix back to the project; patches are always welcome.
If you’re wondering what the developer community tests with, our Jenkins build nodes are currently CentOS 6 and Ubuntu 14.04. There are some Ubuntu 16.04 nodes, but they’re currently offline due to some maintenance issues.
tl;dr: Asterisk runs on Linux. We test on CentOS 6 and Ubuntu 14.04. Anything beyond that is up to you to discover.