Given that the example showed passing artists in as only argument, I think one would consider this supported API and therefore deprecate with a warning instead of raising an error.

IMHO the example is careless and misleading (sorry if the author is here. No personal offence meant).

• It does not make sense to pass all lines explicitly if one does not want to show them. - It might be ok to just demonstrate what a simpleax.legend() does, but that's not great either.
• Given valid usecaseslegend(lines, labels) andlegend(labels), it's misleading to dolegend(lines) where the line objects are converted to str and used as labels.
• This also gives the false impression that the marker lines in the legend are connected to the displayed names. But that's more coincidence. If you had added another artist before, the string of the firstline would be written to that artist in the legend.
• Even though technically acceptedlegend(lines) as used here is not a real-world usecase.Line.__str__ is not useful in a legend at all.

Given these issues, I claim that neither was it the intention of the example to announce passing artists as labels, nor are there hardly any people out there using it in the real world. Even though I'm normally defensive about API changes, I do not consider it necessary to do an API-depreaction dance in this case.