Hardly any native or fluent speaker would say
Die Frage ist in Vergessenheit.
German speakers would rather say:
Die Frage ist in Vergessenheit geraten.
... ist in Vergessenheit geraten is a fixed construction, meaning ... has become forgotten (literally: has come into oblivion or has come to be forgotten).
You could say:
Die Frage ist in Vergessenheit gekommen.
but it would sound unusual. The construction in ... gekommen is generally not used with Vergessenheit. But you can say:
Diese Röcke sind in Mode gekommen
Meaning these skirts have become fashionable or, more literally, these skirts have come to be in fashion.