From the docs here:
Providers requiring registration
In order to use Mapbox maps, you must register.
That links to the generic sign-up page for Mapbox, but they offer a lot of products and it is not clear what you need to register to in order to be able to use their tiles with leaflet-providers.
I haven't looked at the other non-free providers, so I'm not sure whether the same issue may apply.
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