Chayenne appeared in Tibia's 15th Anniversary and not 20th Anniversary, which also contained Cipsoft boss raids. Chayenne is most wanted to reappear as she dropped Chayennes Magical Key which is used in the Realm of Dreams quest. One of the rewards of this quest is a Music Box which can 100% tame certain creatures without using a mounting item. Another fun fact is the key doesn't disappear once used, so you can do the quest on other characters for more Music Boxes. Also, in doing some additional research one of her keys was given to the three winners of Chayenne's Farewell Contest back in 2012. The first place winner actually wrote about a music box.
I don't see Chayenne coming back unless Cipsoft decides to do the opposite of what they've done in the past. There was actually an (official CM response) about this situation during the 20th anniversary:
"Q: Chayenne will not show up?
A: No. As mentioned, former CipSoft members will not appear as bosses."
It's also confirmed the key won't be able to be obtained during Tibia's 25th Anniversary (official CM response)
Please still have hope, for maybe things will change. We are not Cipsoft and cannot give you an official response but just our opinion about Chayenne reappearing or not. Usually, Tibia Anniversary celebrations are announced in early January and sometimes even in December. I recommend checking on Tibia's website in the news section for more details. I know a lot of players who obtained this key are retired or this key is being held hostage as a collector's item.