I like the rust monsters and I like dragons, so it makes perfect sense that I'd like the Rust Dragon! Sadly, it doesn't seem like we'll ever see the return of this noble dragon as there is no mention of it in Fizban's. But that's ok! I can create my own rust dragon, with legendary actions and lair actions!
Rust dragons, at least according to 2nd edition, are the adult form of the rust monster after it goes through a metamorphosis. In 3rd edition, they remove that link, simply stating that rust dragons are rust dragons and rust monsters are rust monsters. Nuts to 3e, my rust dragons are from rust monsters cause I really like the idea of a more insectoid-like dragon.
This is a very dangerous dragon to face, not because of hit point damage, but because it has the chance to ruin expensive armor and weapons. Part of any adventure to bring down a rust dragon is to first suit yourself up for what is going to come, this means that you might include a session where the party has to take on a few bulette and harvest their thick hide as armor or make a deal with an archfey to gain access to darkwood or ironwood, wood as strong as metal but doesn't have the problem of rusting.
Rust Dragon Wyrmling (CR 4), Young Rust Dragon (CR 10), Adult Rust Dragon (CR 16), Ancient Rust Dragon (CR 23), and Rust Greatwyrm (CR 26)