Her husband has left Islam; is she entitled to maintenance during the ‘iddah period?

Yes, you are entitled to maintenance during the ‘iddah, because the separation happened because of the husband, since he left Islam. Imaam al-Shaafa'i (may Allaah have mercy on him) said in Kitaab al-Umm (vol. 6), “If he – i.e., the husband – becomes an apostate (leaves Islam), he should still spend on her (maintenance) – i.e., the wife – during her ‘iddah, because she is not finally divorced until the ‘iddah is over.”