Similarly to couple others… Kashrek was once my tank, keeping me upt to Platinum before he was finally replaced by Kunchen. Now almost never used - not even wars or quests. I think I may have used him on a raid tournament once or twice.
Wu Kong. Once a super star - now replaced totally by Ranvir, sometimes making his way to a war squad or 4* tournament with buff boost.
Cheeshire Cat - was supposed to be my anti-guin, but never served his purpose. Have him maxed but almost never use - unless fighting a war against yellow tanks.
Kelile - oh boy, she was one of my first 4 stars and an amazing hitter early in the game for me. Replaced by Marjana and overshadowed by other powerful red 4 stars, rarely sees a fight now, I think beside the rogue quests.
Chao - purple tanks are rare and even if I meet one - I have better squad for them. Never faced purple tank war in my life. There’s just not enough attacks to use him. Again, he was one of my yellow 4 stars, so I used to run him a lot but he just fallen off quite quickly and I was never too impressed with the guy.
For 5 stars. I plan my mat spending carefully so anyone I took to 4.80 is greatly used. For those who are 3.70 I’d point Guardian Owl. I literally knew he was going to be disappointment the moment I started working for him, but thought he’ll at least help in my 5-6 war team where paired with quickly falling allies. Maybe he filled that purpose for couple wars but having now 40+ heroes maxed the initial need is no longer there. And he really sucks otherwise.