I think so. If Riot and Canonical partner and cooperate on this, it would be beneficial to both. I can't imagine Riot officially supporting anything other than Ubuntu releases though, and only LTS at that.
For Riot it's more of a community thing and street cred. For Canonical, it would drive a lot, possibly a couple million users to their platform. Even if it's free, they want their name out there and the legitimacy. Companies already sign on with Canonical, it's a way to start being a household name and things just snowball from there. It also gets it ready for SteamOS, wherever that lands. It would be a significant project, even to just support Ubuntu LTS. Ideally League will move to embrace the Vulkan API and modify their engine to use that on all 3 platforms, with the existing DX9 support as a fallback on Windows. Their engine needs work and a path off of DX9 eventually, and Vulkan is the best way forward.
The nail in the coffin for me is that Riot is owned by Tencent which is Chinese. China partnered with Canonical to make Kylin, their own OS based on Ubuntu. A free game on a free OS using microtransactions could make a killing in China. Why Tencent wouldn't be working on supporting Kylin/Ubuntu for the massive Chinese market, would be beyond shortsighted.
They're not that shortsighted, I'd put good money down that it's being worked on.
