I'm using matplotlib and label-lines latest 3.10 and 0.7.0 and get the following warning:
DeprecationWarning: The converter attribute was deprecated in Matplotlib 3.10 and will be removed in 3.12. Use get_converter and set_converter methods instead.
if isinstance(ax.xaxis.converter, DateConverter):
Here is the reference.
I had a quick look at this and I think the change is just ax.xaxis.get_converter().