Well, that sucks. I've been a happy Mapbox GL JS user for years, hosting my own tiles and not using their service at all. I even had a few (very small) pull requests accepted into the project. Hopefully the OSS community can maintain a fork of the 1.x codebase.
-edit-
The FAQ says developers can still use self hosted tiles, but also says that usage is now billed per map load, and "a map load occurs whenever a Map object is initialized on a webpage. Each map load includes unlimited vector tiles, raster tiles, and terrain tiles for the length of the map session.".
To me that sound like while I can still use my own map tiles, it would still costs the same as if I used Mapbox hosted tiles.
-edit-
The FAQ says developers can still use self hosted tiles, but also says that usage is now billed per map load, and "a map load occurs whenever a Map object is initialized on a webpage. Each map load includes unlimited vector tiles, raster tiles, and terrain tiles for the length of the map session.".
To me that sound like while I can still use my own map tiles, it would still costs the same as if I used Mapbox hosted tiles.