I’m American (31M), and I don’t use it. I would, however, say “most senior” to refer to the person in a company who has the most professional experience. I might also use it to refer to the oldest person in a group, but that is less common.

It appears to be a regionalism. Seniormost is not in my three favorite dictionaries. The Longman Dictionary of Contemporary English gives -most as a suffix for physical location. -most /məʊst $ moʊst/ suffix [in adjectives] nearest to something the northernmost town in Sweden (=the town that is furthest to the north) the topmost branches of the tree

Definitely not used in Canada/United States. Can't fully confirm for Britain but I've visited twice for conferences and never heard it as well.

Thank you everyone for your answers.