I believe most learners of Japanese were taught that the verbs of the language are classified into 3 groups:
Group 1 / Godan Verbs ([五段動詞]【ごだんどうし】);
Group 2 / Ichidan Verbs ([一段動詞]【いちだんどうし】);
Group 3 / Irregular Verbs;
But apart from those, I've also heard of more groups of verbs other than those 3, like Nidan Verbs or even Yodan Verbs have actually existed.
My question is, do Nidan and Yodan, or even more groups of verbs that I haven’t heard of, still exist in modern Japanese? If so, how do they conjugate? I have been searching for an answer for some time but I don't think there's any.
Also after some researh, I learnt that Nidan verbs have now become Ichidan verbs, and Yodan verbs have become Godan verbs. Is that true?