Is it permissible to suffice with the supplication, "O Allah, forgive all those whom I have backbited," after repenting from backbiting and slander, in order to free oneself from obligation towards them?

It is obligatory to seek forgiveness from whomever you have backbitten, if they are alive and accessible. Otherwise, it suffices to supplicate for them and to mention their good qualities in the same places where you mentioned their faults. If the number [of people backbitten] is large and seeking their forgiveness is impossible, then it is hoped that supplicating for them in the plural form, without specifying or naming individuals, will suffice.