The fashion of bell-bottomed trousers has become out of date.

Solution (By Examveda Team)

One word substitute is Obsolescent.

Preposterous: contrary to reason or common sense; utterly absurd or ridiculous.
Debilitating: (of a disease or condition) making someone very weak and infirm.
Obsolescent: becoming obsolete.
Obstreperous: noisy and difficult to control.