Based on this question, I was curious if there is an actual term that describes how words' meanings change or become deprecated over time.
It's known as semantic change, semantic shift, or semantic progression.
Coincidentally, gay is mentioned on that Wikipedia article:
Gay - Originally meant feelings of being "carefree", "happy", or "bright and showy"; it had also come to acquire some connotations of "immorality" as early as 1637. The term later began to be used in reference to homosexuality, in particular, from the early 20th century, a usage that may have dated prior to the 19th century.