If you have plenty of room in your wardrobe then of course it's fine to keep them there, but if you don't?
If you know you won't actually wear them again but still want to keep them, I think you treat them as memorabilia rather than as clothes, and put them somewhere other than in the wardrobe.
They could go in a box or second wardrobe to be brought out and looked at occasionally. Or you could make a decorative feature of them - hung on the wall, or on a pretty hanger on the back of the bedroom door or the front of the wardrobe door, or, my own preferred option, they could be photographed - on you, or on a hanger - this certainly takes up less space!
If you like keeping things this way click here for Gretchen Rubin on organising keepsakes (it's just not for me - more boxes in the loft?!)