Update: The community fork discussed below became MapLibre GL JS, now the standard open-source alternative to Mapbox GL JS. Stadia Maps is an active MapLibre contributor, and our tiles work with it out of the box — see our Switch from Mapbox guide.
Yesterday (Dec. 8th, 2020), in conjunction with the v2 release of the Mapbox GL JS library, the code was re-licensed to require an active commercial license and subscription agreement with Mapbox. As a result, if you are currently using Mapbox GL JS v1 with Stadia Map tiles and upgrade to v2, you will need to pay Mapbox (as well as Stadia Maps) for map usage.
For now, please continue using Stadia Maps' map tiles with version v1.13 or below. Since these versions are fully open source and freely licensed, you will not incur any additional charges and your maps will continue to function as expected.
Stadia Maps is actively working to build consensus towards a community-driven, fully-open and free fork of Mapbox GL JS v1, and we will continue to support and collaborate on these efforts going forward. If you’re a developer or an organization utilizing the Mapbox GL JS or GL Native libraries and are looking for an alternative, feel free to reach out. We will continue working together for a more open and more accessible open mapping ecosystem.