Despite being the OU Pokemon, it is untiered on Smogon because it is not one of the Pokémon from CAP to make it to the battle simulator (being Pokemon Showdown which includes the CAP Fakemon such as Pajantom or Syclant) Smogon uses. If it was tiered it would be potentially OU (Assuming Gen VII) due to having a high speed stat, Shell Smash and Spore.
Despite being the OU Pokemon, it is based on Talonflame who is no longer in OU due to the Gale Wings nerf and not having Gale Wings as an ability.
Contrary to it's pokédex description, Smoxilon has one of the widest movepools in the game, making it unpredictable in exactly what moves it will use.
Its shiny is based on Koffing's shiny.
Smoxilon and its pre-evolution Smogaroben are based on Smogon. Smoxilon is a purple Talonflame with a Koffing as its head with a gaseous tail. Its movepool also consists of moves commonly used on Smogon sets (Stealth Rock, Scald, etc.) and its species being the OU Pokémon is an obvious reference to Smogon's OU tier.
Smoxilon is a combination of Smogon, toxic, talon and Talonflame.