A wife is menstruating or in postnatal confinement. Is she still entitled to her turn?

Yes. A wife who is menstruating, in postnatal bleeding or ill is still entitled to her night in the rotation, because the purpose of the rotation is affection and companionship, not merely intimacy.
The husband still spends the night with her, keeping her company, helping and being affectionate within permitted limits, without intercourse during menstruation and postnatal bleeding (Surah al-Baqarah 2:222). Postponing her turn or moving it to another wife on the grounds that he cannot be intimate with her is an injustice. Indeed, the confinement period after childbirth is when a wife most needs support. See Fiqh of polygyny.