My pet lives strictly indoors. Do they still need deworming?

はい、必要です。
完全に室内で飼っている場合でも、寄生虫に感染する可能性はあります。

Even indoor pets can still be exposed to parasites through mosquitoes that enter the home, parasite eggs brought in on shoes or clothing, or small carrier insects that may transmit tapeworms if accidentally eaten by the pet.