Which word would best describe ideas or practices that are no longer in use?

The term "obsolete" is the most suitable choice for describing ideas or practices that are no longer in use. The word specifically conveys the notion of something being outdated, outdated, or no longer relevant due to the advent of newer alternatives or advancements. In historical or technological contexts, something categorized as obsolete has been replaced or surpassed by more effective or modern approaches, rendering it ineffective or impractical in its original form.

In contrast, "contemporary" refers to something that is current or belonging to the present time, indicating active relevance rather than a lack of usage. "Modern" similarly denotes something that is characteristic of the present, particularly relating to the latest techniques or styles, which again does not align with the idea of being out of use. "Transitional" suggests a state of change or movement from one condition to another, emphasizing a period of transformation rather than the finality implied by obsolescence. Therefore, "obsolete" clearly encapsulates the concept of being out of use, making it the correct and most precise answer.