Formal, often used in official or legal contexts such as obituaries, reports, or announcements. Not used in casual conversation.
It was announced that there were three deceased in the accident.
Compile a list of the deceased.
死亡者 simply means 'fatality' or 'person who died', without the formal, respectful nuance of 物故者.
物故 (ぶっこ, 'death') + 者 (しゃ, 'person'). 物故 is a Sino-Japanese compound meaning 'passing away', used in formal contexts.