Underleveled Pokemon are currently in a state of reworking. As it stands, the validator will accept things like Rhyperior, Florges, and Crobat (all of whom evolve to their final stage via something other than a level threshold) at level 1, when no game makes such an encounter legal, and I already loaded in the table of underleveled encounters for every generation but Showdown doesn't currently do anything with that table of levels. I imagine that somewhat soon, the logic for handling the encounter lists will be incorporated, solving both the cases of "these Pokemon should be available at a lower level than their natural evolution level" and the cases of "despite not being gated to a specific level threshold for evolution, these Pokemon should require a certain minimum level simply due to the realities of the prevo's available level range".