Well, some bugs aren‘t really features but have existed for so long that they basically are or were. For example since his release Ornn was bugged in the sense that Tenacity reduced the cc duration of his brittle procs which are actually considered knockups/knockbacks. Ornn players just had to live with this until Riot finally fixed it sometime earlier this year.
Then there are also bugs that seem broken but aren‘t broken in practice. There were bugs that allowed Viego to gain infinitly scaling attac speed every time he killed and posessed a Kog‘Maw. In theory you could be running around with 90 aps but realistically you‘d get like an additional 10% attack speed max.
Then there are bugs like on the current PBE where you can buy and undo the item with Attack speed, AD, health and Ultimate ability haste on repeat to gain permanent 500 AH on your ultimate. Bugs like this shouldn‘t even be out for 7 days on the PBE let alone be turned into features.
Depends on what you consider ‚Never played‘. Like 40% of the chamions weren‘t picked a single time at all during this years entire run of worlds. But in SoloQ/Normals there is currently no champion played by noone. But for champions like Taric, Rell, Ivern, Ryze or Azir it feels like in regular play there are like 5 people who play those champions, even if some of them are basically pick or ban at worlds.