With regards to giving divorce in the state of menstruation or nifaas; it is talaq bid'i (innovated talaq) and the most correct opinion of scholars is that it is not counted.

Talaq bid'i; is divorce given to a menstruating woman or divorce to a wife in her state of purity in which her husband had marital relations with her. It is unanimously agreed by all scholars that such talaq is harām based upon the following verse;

‘O Prophet! when ye do divorce women, divorce them at their prescribed periods' [65:1]

And also when Ibn Umar divorced his wife while she was menstruating, the prophet got angry at him and ordered him to take her back and to give her divorce while she is in her purity period without having any marital relations with her. The prophet then said;

‘this is the iddah period in which Allah has ordered for the women to be issued divorce' [reported by all sunan except for Tirmidhi]

The intent behind ordering Ibn Umar by the prophet was to take back his wife, meaning to his house, and this does not mean that divorce was counted upon him and for this reason the scholars have disagreed in the divorce given by Ibn Umar as counted by the prophet or not?

This is why they say; is talaq bid'i counted alongside been prohibited, or is it seen as void?

The correct opinion is that it is not counted because it is an innovated talaq which has been prohibited, and prohibition leads to nullification of the action. Ibn Taymiŷah said;

‘the principal which the Salaf (pious predecessors) and Fuqahā were upon that: any worship or transaction performed in a way which is forbidden, is not binding to be accepted, even if some of the ahlul kalām (theologians) may disagree, but the correct stance is with the salaf and with Fuqahā because thesahāba and tābi'een would consider invalidation of an act of worship or transaction based upon the prohibition set by the Allah and this is mutawātir from them.

And also if (the above mentioned) is not an evidence upon its invalidity (talaq bid'i), then there is no evidence to prove that the Legislator has approved of something from Fasād (invalidation).

Also the Legislator (Allah) would not prohibit something except that it consisted of pure harm or majority of it was harm......'

Also the hadith of the prophet, narrated by Ayesha;

‘Whosoever invents in our religion that which is not a part of it will be rejected' agreed upon.

Meaning that action would be rejected, and this illustrates that any action made prohibited in Islam then it is void without having any effects. Hence, from those prohibited actions is talaq bid'i.