Providers requiring registration
Mapbox
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.
From the docs here: https://github.com/leaflet-extras/leaflet-providers
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.