Izzymon
Donphan has not been banned. Initially it was probably deemed mostly unviable in this current metagame. In a metagame where Excadrill is free, a large question mark is placed above Donphan's head. This along with the fact that Defog also exists as an option to get rid of hazards and the lack of good reasons to use Donphan on a team outside of the SR + Rapid Spin. It has a poor defensive typing, subpar specially defensive bulk and is very limited in the role it can play. If you opt for offensive investment then it gets worn down extremely quickly. However, if you go for defensive investment then its EQ/Ice Shard do barely any damage. Moreover, Donphan also lacks ways to get past the common spinblockers in the metagame such as Gengar, Jellicent and Gourgeist without relying on gimmicky options such as a Choice Band.
Despite these shortcomings, Alexwolf permitted it to gain an analysis a few pages back when he was brought up previously: