So basically SQL server on linux is supported until the project isn't fun to work on anymore which then it will be left to rot and eventually support dropped entirely.
I'd say it will stay as long as commercially viable, which it very much is... I'd imagine a lot of this effort was to get SQL services on azure closer to the metal than running on top of Windows.
I don't think it's going anywhere for a while... and I rarely trust MS with anything cross platform.
