Can I use an engine in unrated games?

The short answer is Yes. Unrated games, and unrated tactics are the only exceptions to the normal rules. Since these games and tactics are not rated, and do not affect the rating of any other players on the site, you are free to experiment with whatever tools, and learn however you choose! However, if you are going to use assistance against another person, you must let them know beforehand!

Our Policy on Fair Play has now been amended and it is no longer allowed to use outside assistance when playing a human opponent EVEN if it is Unrated: https://www.chess.com/legal/fair-play

Slight edit for clarity: "normal" rules instead of "above" rules since there's nothing "above" here, and also "please let them know" changed to "you must let them know" since people can actually get banned for cheating in unrated games without first getting agreement from their opponent.

TOS update : it is now forbidden to cheat in unrated games, even if the opponent agrees with it.

Where is that stated?

That may be controversial. Some people like to play unrated centaur games. There's even a largely inactive club dedicated to running unrated centaur tournaments: https://www.chess.com/club/advanced-chess

The fair play policy doesn't specifically make an exclusion for unrated games; it just doesn't mention them: https://www.chess.com/legal/fair-play

Any change in policy on unrated games would need stated very clearly with a very clear announcement.

From the updated policy:

NOTE: This Fair Play Policy applies to your use of our Services; however, it does not apply to your games that are not against other users, such as in the game mode versus Chess.com computer (i.e., non-human, virtual) “bots“

The exception was explicitly removed 

Correct, the only exception is non-human play. So, Whether you're in rated or unrated play, you are not to use engines against other human opponents as this now breaches our Fair Play Policy.

It would be good to explicitly state that. Unrated games have always been an exception (and are still implied to be an exception here https://support.chess.com/article/648-what-do-i-need-to-know-about-fair-play-on-chess-com).

There has occasionally been discussion in the Cheating Forum where people have suggested playing unrated games where a lower-rated player does limited engine use to test what kind of stats that generates, or simply as a form of an odds match. That would now be forbidden, and the change has gotten no publicity.

This is a pretty major change to a long-standing policy, and it needs to be made explicit and announced very clearly and publicly. Removing an exception is not sufficient to remove potential confusion.

Otherwise I can easily see a large number of people getting banned through ignorance when they had no intention of cheating and everything had the full agreement of their opponent.