I ranked all 153 Pokemon Type combinations by similarity in Excel (link in comments) : r/pokemon

Unused type combinations Gen VI : r/pokemon

Instead of unused type combos, which long-neglected type combos would you like to see re-used? Here is a list of type combos that have not seen a new Pokemon in over 20

Every Type Combination That Doesn't Exist RANKED

All Pokemon Type combination by ZarxielZerg on DeviantArt

Of the 306 possible Pokemon dual type combinations, how many have not been used? - Quora

Pokemon Fans Are Suggesting 'Mons For The 9 Unused Type Combinations

Unused type combinations Gen V by AdeptCharon on DeviantArt

Every New Type Combination in Scarlet and Violet, and a Few That Should Have Been

The Rarest Pokemon Type Combinations

Pokemon Type Combination Frequencies, Updated for Generation 7