Doesn't have nudity so far, the most I've seen is characters in their underwear. The R18+ is probs less about porn (though there are some written-only sexy times but overall they're few and far between) and more about the darker themes the devs probs feel aren't appropriate for minors:
"Echo contains themes of an adult nature, including violence, torture, drug usage, and harsh language - you must be 18 or older to play."