Important utility of supports typically come from healing/shielding allies, buffing allies, providing CC. Teemo doesn't do either of the first two at all and only has very weak CC. His map awareness helps to make up for that, but he's providing it regardless of role
teemo is an all around sort of champ who does many things. It is not the actual role of a support champ to not take kills and gold. This just happens to be a very efficient strategy.
Support: Champions whose skills are meant to directly aid the rest of the team. They can either babysit a champion to make laning easier for them by providing healing, buffing allies, applying debuffs to the enemy team, or a combination of the above. The most common ability for a support champion is providing an ally with a shield of some kind. One of the main goals of a support is to let their lane partner farm minions for gold, as supports can buy key gold+ items to get gold for themselves, as well as controlling the map with wards, warding all key areas of the map. These champions usually rely on their skills more than their auto-attack.
Teemo fits this for the most part, he wards and applies two different debuffs.
It's also worth noting that, in the current meta, supports are expected to have limited income. Teemo has no late game presence without having quite a bit of farm. Teemo doesn't fit into the support role at all. He can be decent in a kill lane, but there are support champions who do that better. He can harass well, but their are supports who do that better. Most of the champs who fill those roles also don't fall apart late game on a supports income.