- GRASS has had that since the 80s: https://grass.osgeo.org/grass72/manuals/addons/r.traveltime....
- ESRI has had those in web form since the 90s: https://doc.arcgis.com/en/arcgis-online/analyze/create-drive...
- GeoTrellis has that working in an open source project on top of a Spark cluster: https://transit.geotrellis.io/travelshed.html
What I meant is the ability to generate them via an API without having to handle any GIS / routing data yourself and this for any city.
Handling these kind of services takes a lot of time, especially if this is only a sub-part of a more generic product.
Thanks for the alternatives though
The third option is faster than anything else, but would require you to load planet-osm.