Mapbox made their map renderer, Mapbox GL, closed-source in 2020. I left in 2017 so wasn't involved in this decision, but I worked on Mapbox GL and know the rest of the narrative. Mapbox was part of a wave of open-core startups that released the core of their products as open source and were able to make a decent business as well as do a good thing for the world. The amount of time and money that Mapbox put into open source code is pretty unrivaled. But, big competitors started just running the same versions of the same software for cheaper and the vibes shifted and so decisions like this one was made.
MapLibre started as a fork of Mapbox GL 1.x, the open source version, and has evolved a lot since then but still owes a lot to the original. The maintainers are very cool about this and the repo makes the lineage clear. But there's a narrative of rebelling against the corporation by using a version of the code open sourced by the corporation. That narrative hurts because I saw both the effort put into the inventions behind Mapbox GL, as well as appreciated that a company committed to funding and open sourcing that stuff despite norms and investor pressures. If there's a villain in the story, it's the companies that copy open source code into their products, never contribute back, and undercut smaller companies with their hyperscaler advantages.
So, the purpose of this too-long footnote is to make it clear that I appreciate both the fact that Mapbox paid a ton of people to release a ton of world-changing code that meaningfully shifted the industry forward, and that the MapLibre team is working on and improving an open source project partially based on that code.