I know I'm still behind on looking at this, but just to mention why I haven't yet merged it, I want to stare at this closer, as I'm somewhat suspicious of the types here -- specifically, it's likely true that validator_value is always a container for any builtin JSON Schema keywords, but certainly not in general (i.e. someone can invent some other one).

And similarly below I'm slightly suspicious of the schema_path business.

Not to say any of it looks wrong, but I want to convince myself if it needs any additional hardening.

And of course again thanks for the thought and PR, I'll get to it sometime soon I hope (days not weeks).

If assuming that only oneOf and anyOf can have non-empty context, then schema_path is seemingly always an integer because it was produced from enumerating https://github.com/python-jsonschema/jsonschema/blob/v4.22.0/jsonschema/_keywords.py#L340
This also implies that yes, validator_value may be something that can be enumerated, but without len.

Alternatively, assuming that best.context has errors in strict order of .schema_path (see also the same algorithms _keywords.anyOf | oneOf):

        best_in_subschemas = []
        for error in best.context:
            index = error.schema_path[0]
            if index == len(best_in_subschemas):
                best_in_subschemas.append(error)
            else:
                prev = best_in_subschemas[index]
                best_in_subschemas[index] = max(prev, error, key=key)