기에 is still used but its use in speech is somewhat waning, I think.
In speech, its shorter form -기 is used more often, -기에 seems to be more for emphasizing the “purpose” aspect of it.
The example 이 아파트는 한 사람이 살기에 적당하다 is fine, but one could also say “이 아파트는 한 사람 살기 적당해” without much loss of meaning.
-기 is also broader in meaning, in that it can mean both -기에 (purpose, utility) and -기가 (situation, disposition). For example, 살기 좋은 곳 can mean both 살기에- and 살기가 좋은 곳, while 하기 싫은 일 is strictly 하기가 싫은 일 (a task I’m loath to do) for which -기에 won’t make sense. These are very common phrases.
-기에 also can mean “because”, or “since”, as in “내가 모르기에 …” (because I do not know…). Even in this sense, it is mostly used in text, since in speech there are better forms, like “몰라서…” “몰랐기 때문에”, etc.
So -기에 seems to be used mostly in deliberate speech and text.
Update
The above explanation is not quite right because it is too narrowly focused on -기에 and -기 without the big picture.
-기 is actually just an ending of a noun form (하다 → 하기, 높이다 → 높이기), so it should be explained with that in mind.
살기에 좋다 is thus + 에 + , where the noun has the -기 ending. The noun can have different ending too, like in 속쓰림에 좋다 (good for heartburn/indigestion), where it has -임 ending.
So the big picture of related expressions are:
- 살기에 좋다 nice to live (in) - purpose is emphasized
- 살기가 좋다 nice - neutral (no special connotation)
살기(에)는 좋다 nice (but I don’t know about other things)
살기(에)도 좋다 nice (in addition to other things)
살기(에)만 좋다 nice (but not so for other things)
…
Of these, the asterisked ones are very often shortened to just “살기” because they imply the most common, typically usage as opposed to the rest which have special connotation.
It got very long, but the first explanation just didn’t seem right