What is meant by Al-Jarḥ wa al-Taʿdīl (discrediting and accrediting of Hadith narrators), is it permissible to say that so-and-so is “discredited” so his narration is not to be accepted, and who is its standard-bearer in our current era?

It is permissible to say, "So-and-so is impugned, so nothing should be taken from him," if the intent is to impugn narrators and witnesses. This is permissible by consensus and obligatory to safeguard the Shari'ah. There is no bearer of the banner of al-jarh wa al-ta'deel (impugnment and accreditation) in our current time. Those who speak on it rely on the sayings of the predecessors, and it is not correct to judge anyone among them as bearing this banner.