Just found this and I've got a bit of new info that could be useful.
As mentioned, Alolan Muk falls into the same category as Drapion and Skuntank, but Genesect should also be included with the Bug/Steels.
While in the same vein as Rotom-Wash, Weezing with Levitate is only weak to Psychic and Seaking with Lightning Rod is only weak to Grass, but for historic reference, Bronzong had only 2 weaknesses (prior to Steel's loss of Ghost and Dark resistances in Gen 6), each of which was neutralized/negated by one of it's abilities (Heatproof neutralized Fire and Levitate negated Ground) and Ghost/Dark had 0 weaknesses prior to Fairy's introduction in Gen 6.
As far as Pokemon with single weaknesses based on typing alone (not ability) there are only 5 possibilities with the current type chart and we have 4 of them (Bug/Steel, Water/Ground, Poison/Dark, and Ghost/Dark); the 5th would be a Normal/Ghost and would be weak to Dark only.
As side mention, pure Electric and Normal Pokemon naturally only have 1 weakness each (Ground and Fighting respectively) and there is only 1 Pokemon family with no weaknesses still to this day: Eelektross (because pure-Electric with Levitate), but is irrelevant due to being cut from Gen 8 at this time.