There's a difference between not being desirable and there being better options, League doesn't have hard counters like Dota, so it gets restricted by it's tier list:

S Tier: you have 3-4 Champions per Role that are S tier, usually overpowered champions in a particular patch, thanks to some balance changes or recent item interaction

A Tier: then you have the A tier pool which is around 10~15 champions that vary on how efficient they are or how safe of an option they are (no point in picking X champion that might be good early but struggle late game, when you can pick Y champion that has a smoother early to late game scaling)

B Tier: then you have a huge pool of B tier champions that are too specific in what they do well, maybe they are amazing in a particular comp but if something goes wrong they are useless (and there's no powerful items to make them useful late like in dota)

C Tier: you also have a small pool of trash tier champions which are really competitively nonviable because they suck in coordinated play, like Master Yi or pre-rework Eve Jungle, they are both pretty good in SoloQ environment, but in competitive play where teams are coordinated and can keep tabs on where they are and can easily shut them down they are just shit (and like in the previous tier, you can't counterbalance being shutdown by smartly itemizing like in Dota)