It also means it's easy to create reference cycles that never get deallocated. Especially in callback anonymous functions.
A simple rule of thumb is that if anything has a parent/child relationship, the child should use the weak keyword (or even “unowned” if you’re certain about object lifetimes) when referring back to the parent. That and things like hand-rolled doubly linked lists, but the standard library already has that so who cares.
In C#, these aren't the exact same as a reference cycle, but the end result is the same, a memory leak: https://learn.microsoft.com/en-us/dotnet/desktop/wpf/events/...
AppCode was abandoned because Apple is impossible to work with and never provided up to date semantics and language definitions for Swift, keeping everything under wraps and making every single Swift release a race to suddenly implement whatever new keyword they made up this week.
Fleet is more of a VSCode. Probably using the LSP these days, and it's good enough if a simple text editor is what you need.
I think the reason they abandoned it was trouble keeping up with new iOS SDK features getting shadow dropped as part of new Xcode betas and iOS devs wanting to immediately prepare their apps to use those features, not anything to do with Swift language releases. Also things like having to recreate interface builder, then support auto layout in it, and then later SwiftUI previews, etc. Again all iOS platform stuff, not Swift language stuff.