This is like Jax and dodge all over again. Having to buy a specific item to counter a single champion is wrong. It removes flexibility of build, and it's a bad trend to get into. Imagine if old jax was still in, and you are playing ADC. That means in a game with Jax and Teemo, you're compelled to buy Sword of the Divine and Lightbringer. Not required, granted, but very good reasons to buy.
And what if more champions end up like this? You end up with 5 items in your build dedicated to countering specific champions, and as a result, reducing your effectiveness.
If the issue is that Teemo's mushrooms are too hard to counter, then change mushrooms. Don't just nerf the damage into oblivion, but change the base mechanic to fit in with the desired effect. Could be as simple as, make them visible, but require 5 hits to destroy. Or whatever.
There's no point having lots of different items in the game, if some of the are 'required'.