The docs of `git merge` do indicate acceptably clearly that multiple commit-ish can be given.
I expect it's really more that "octopus" merge are usually of limited use, most projects are not structured as a distributed "tree of trees" the way the linux kernel is, where merging multiple downstreams regularly is a normal workflow and thus octopus merges make a lot of sense.
Joys of having Linux and git both written+managed by Torvalds:), although I do wonder if that points to to a shortfall in docs/UI.