I would like to know whether ignoscere coexisted with other Latin verb or verbs of the same or similar meaning since ignoscere was, as far as I know, a cultism, or wasn't it?. Besides, I would like to have some historical information: was ignoscere still in used during the Middle Ages and until Latin was less and less talked and written? Thanks for your help
A cultism is a Latin borrowing into a Romance language - there can be no cultisms in Latin.
ignōscere is part of the core vocabulary of Latin in every period, and the 742nd most common word classically. It's a word as indispensible to the language as 'forgive' is to English. You should have no qualms about using it.
I've already mentioned one word of similar make-up and meaning; there's another related expression,
veniam dare "to give permission, to allow, to let off", literally "to give leave". The action is noticed, unlike with
ignōscere, but nevertheless allowed, excused, indulged etc. The person is allowed to walk away without consequence. For example a boy in class would say
dā veniam, magister in order to ask to go to the bathroom.
excūsāre "to absolve from blame" is also somewhat similar in being used to ask for something like forgiveness. For example you'd say
excūsēs rogō when letting someone know you can't come to dinner, or if you've failed to fulfill an obligation - you're asking not to blame you, since you have a reason, an excuse.