Reasonable base layers for maps are table stakes for geospatial apps, so it's great to see Protomaps getting some attention. Requiring an expensive MapBox subscription (pay per tile render) for popular apps just meant that it became prohibitively expensive to run side projects with maps.
If you're interested in cartography, Facebook created the Daylight Map Distribution [1] project. They analyzed satellite imagery, outlined buildings and roads, and published them for map makers. It significantly improves coverage in rural areas and non-Western countries.