Do you have another example? To shout at is a phrasal verb, so the preposition is limited in mobility, so "This is the person at whom was shouted" loses the sense of the verb. It might be said that This is the person who was being shouted at isn't prepositional stranding as such, as the preposition is part of the verb.
Where a preposition is used normally:
"Each family group seems to have had a senior person to whom was allotted a tract of land on which the family lived and worked."